This means Divine Voice. An example of its use is in resolving the dispute between Beit Shammai and Beit HIllel (see here: Beit Shammai and Beit Hillel).
Regarding its validity
Talmud, Seder Moed, Megillah 32a, 11:
ואמר ר’ שפטיה אמר ר’ יוחנן מנין שמשתמשין בבת קול שנאמר (ישעיהו ל, כא) ואזניך תשמענה דבר מאחריך לאמר והני מילי דשמע קל גברא במתא וקל איתתא בדברא והוא דאמר הין הין והוא דאמר לאו לאו
And Rabbi Shefatya said that Rabbi Yoḥanan said: If one was deliberating about whether to do a certain action, and a Divine Voice indicated what he should do, from where is it derived that one may make use of a Divine Voice and rely upon it? As it is stated: “And your ears shall hear a word behind you saying: This is the way, walk in it” (Isaiah 30:21). The Gemara comments: This applies only when one heard a male voice in the city, which is unusual, for men are usually found in the fields, or when one heard a female voice in the fields, for women are generally not found there. Since the voice is unusual, one need not doubt it and may rely upon it. And that applies when the voice repeated its message and said: Yes, yes. And that also applies when the voice said: No, no.
The Handbook of Jewish Thought by Rabbi Kaplan, 6:6:32:
A bath kol is like a voice heard in the mind. Although it sometimes sounds like a voice from heaven, it is a prophetic, rather than a physical voice, and it is only heard by those for whom it is intended. It was a voice often heard by neophytes not yet ready for privacy. Regarding the bath kol it is written, “Your ears shall hear a voice behind you” (Isaiah 30:21).
Guide for the Perplexed, 2:42:
The bat-kol (prophetic echo), which is so frequently mentioned by our Sages, and is something that may be experienced by men not prepared for prophecy.
Distinguishing it from evil whispers
Basically it’s just that the Bat Kol cannot contradict the Halakha, or even upset the majority of Rabbi-s, as the Torah is “Not in Heaven.” This is illustrated in the story of the Oven of Akhnai below, where Rabbi Eliezer’s invocation of Bat Kol was denied by the Rabbi-s.
Case 1: Oven of Akhnai
The phrase “Lo Bashamayim Hi” (Not in Heaven) was derived from this story. This is to say that the meaning of the Torah is not to be uncovered by prophets, or even by the voice of God, but is to be interpreted by humans. Likewise, the Midrashic reading of Deuteronomy 17:11 is that God’s will was revealed at Sinai its Grundnorm (basic norm), and he entrusted the interpretation of His will to the Sages.
Mishnah, Seder Tahorot, Mishnah Kelim, Page 5, Line 10 (Referenced in the discussion below)
חֲתָכוֹ חֻלְיוֹת וְנָתַן חֹל בֵּין חֻלְיָא לְחֻלְיָא, רַבִּי אֱלִיעֶזֶר מְטַהֵר, וַחֲכָמִים מְטַמְּאִין. זֶה תַנּוּרוֹ שֶׁל עַכְנָאי. יוֹרוֹת הָעַרְבִיִּין שֶׁהוּא חוֹפֵר בָּאָרֶץ וְטָח בְּטִיט, אִם יָכוֹל הַטִּיחַ לַעֲמוֹד בִּפְנֵי עַצְמוֹ, טָמֵא. וְאִם לָאו, טָהוֹר. וְזֶה תַנּוּרוֹ שֶׁל בֶּן דִּינָאי:
If he cut the oven up into rings, and then he put sand between each pair of rings, Rabbi Eliezer says: it is clean. But the sages say: it is unclean. This is the oven of Akhnai. As regards Arabian vats, which are holes dug in the ground and plastered with clay, if the plastering can stand of itself it is susceptible to impurity; Otherwise it is not susceptible. This is the oven of Ben Dinai.
Context
Talmud, Seder Nezikin, Bava Metzia, 58b:6:
מַתְנִי׳ כְּשֵׁם שֶׁאוֹנָאָה בְּמִקָּח וּמִמְכָּר, כָּךְ אוֹנָאָה בִּדְבָרִים. לֹא יֹאמַר לוֹ: בְּכַמָּה חֵפֶץ זֶה? וְהוּא אֵינוֹ רוֹצֶה לִיקַּח. אִם הָיָה בַּעַל תְּשׁוּבָה, לֹא יֹאמַר לוֹ: זְכוֹר מַעֲשֶׂיךָ הָרִאשׁוֹנִים. אִם הוּא בֶּן גֵּרִים, לֹא יֹאמַר לוֹ: זְכוֹר מַעֲשֵׂה אֲבוֹתֶיךָ, שֶׁנֶּאֱמַר: ״וְגֵר לֹא תוֹנֶה וְלֹא תִלְחָצֶנּוּ״.
MISHNA: Just as there is a prohibition against exploitation [ona’a] in buying and selling, so is there ona’a in statements, i.e., verbal mistreatment. The mishna proceeds to cite examples of verbal mistreatment. One may not say to a seller: For how much are you selling this item, if he does not wish to purchase it. He thereby upsets the seller when the deal fails to materialize. The mishna lists other examples: If one is a penitent, another may not say to him: Remember your earlier deeds. If one is the child of converts, another may not say to him: Remember the deeds of your ancestors, as it is stated: “And a convert shall you neither mistreat, nor shall you oppress him” (Exodus 22:20).
Talmud, Seder Nezikin, Bava Metzia, 59a:8 (Same Sugya):
אָמַר רַבִּי אֲבָהוּ: שְׁלֹשָׁה אֵין הַפַּרְגּוֹד נִנְעָל בִּפְנֵיהֶם: אוֹנָאָה, וְגָזֵל, וַעֲבוֹדָה זָרָה. אוֹנָאָה, דִּכְתִיב: ״וּבְיָדוֹ אֲנָךְ״. גָּזֵל, דִּכְתִיב: ״חָמָס וָשֹׁד יִשָּׁמַע בָּהּ עַל פָּנַי תָּמִיד״. עֲבוֹדָה זָרָה, דִּכְתִיב: ״הָעָם הַמַּכְעִיסִים אוֹתִי עַל פָּנַי תָּמִיד וְגוֹ׳״.
Rabbi Abbahu says: There are three sins before whose transgressors the curtain [hapargod] between the world and the Divine Presence is not locked; their sins reach the Divine Presence. They are: Verbal mistreatment, robbery, and idol worship. Mistreatment, as it is stated: “And a plumb line in His hand”; robbery, as it is stated: “Violence and robbery are heard in her, they are before Me continually” (Jeremiah 6:7); idol worship, as it is stated: “A people that angers Me before Me continually; that sacrifice in gardens, and burn incense upon bricks” (Isaiah 65:3).
Discussion
Talmud, Seder Nezikin, Bava Metzia 59a:12 - 59b:5 (Same Sugya):
תְּנַן הָתָם: חֲתָכוֹ חוּלְיוֹת, וְנָתַן חוֹל בֵּין חוּלְיָא לְחוּלְיָא – רַבִּי אֱלִיעֶזֶר מְטַהֵר, וַחֲכָמִים מְטַמְּאִין.
§ Apropos the topic of verbal mistreatment, we learned in a mishna there (Kelim 5:10): If one cut an earthenware oven widthwise into segments, and placed sand between each and every segment, Rabbi Eliezer deems it ritually pure. Because of the sand, its legal status is not that of a complete vessel, and therefore it is not susceptible to ritual impurity. And the Rabbis deem it ritually impure, as it is functionally a complete oven.
וְזֶה הוּא תַּנּוּר שֶׁל עַכְנַאי. מַאי עַכְנַאי? אָמַר רַב יְהוּדָה אָמַר שְׁמוּאֵל: שֶׁהִקִּיפוּ[הוּ] דְּבָרִים כְּעַכְנָא זוֹ, וְטִמְּאוּהוּ. תָּנָא: בְּאוֹתוֹ הַיּוֹם הֵשִׁיב רַבִּי אֱלִיעֶזֶר כׇּל תְּשׁוּבוֹת שֶׁבָּעוֹלָם, וְלֹא קִיבְּלוּ הֵימֶנּוּ.
And this is known as the oven of akhnai. The Gemara asks: What is the relevance of akhnai, a snake, in this context? Rav Yehuda said that Shmuel said: It is characterized in that manner due to the fact that the Rabbis surrounded it with their statements like this snake, which often forms a coil when at rest, and deemed it impure. The Sages taught: On that day, when they discussed this matter, Rabbi Eliezer answered all possible answers in the world to support his opinion, but the Rabbis did not accept his explanations from him.
אָמַר לָהֶם: אִם הֲלָכָה כְּמוֹתִי – חָרוּב זֶה יוֹכִיחַ. נֶעֱקַר חָרוּב מִמְּקוֹמוֹ מֵאָה אַמָּה, וְאָמְרִי לַהּ: אַרְבַּע מֵאוֹת אַמָּה. אָמְרוּ לוֹ: אֵין מְבִיאִין רְאָיָה מִן הֶחָרוּב. חָזַר וְאָמַר לָהֶם: אִם הֲלָכָה כְּמוֹתִי – אַמַּת הַמַּיִם יוֹכִיחוּ. חָזְרוּ אַמַּת הַמַּיִם לַאֲחוֹרֵיהֶם. אָמְרוּ לוֹ: אֵין מְבִיאִין רְאָיָה מֵאַמַּת הַמַּיִם.
After failing to convince the Rabbis logically, Rabbi Eliezer said to them: If the halakha is in accordance with my opinion, this carob tree will prove it. The carob tree was uprooted from its place one hundred cubits, and some say four hundred cubits. The Rabbis said to him: One does not cite halakhic proof from the carob tree. Rabbi Eliezer then said to them: If the halakha is in accordance with my opinion, the stream will prove it. The water in the stream turned backward and began flowing in the opposite direction. They said to him: One does not cite halakhic proof from a stream.
חָזַר וְאָמַר לָהֶם: אִם הֲלָכָה כְּמוֹתִי – כּוֹתְלֵי בֵּית הַמִּדְרָשׁ יוֹכִיחוּ. הִטּוּ כּוֹתְלֵי בֵּית הַמִּדְרָשׁ לִיפּוֹל. גָּעַר בָּהֶם רַבִּי יְהוֹשֻׁעַ, אָמַר לָהֶם: אִם תַּלְמִידֵי חֲכָמִים מְנַצְּחִים זֶה אֶת זֶה בַּהֲלָכָה, אַתֶּם מָה טִיבְכֶם? לֹא נָפְלוּ מִפְּנֵי כְבוֹדוֹ שֶׁל רַבִּי יְהוֹשֻׁעַ, וְלֹא זָקְפוּ מִפְּנֵי כְבוֹדוֹ שֶׁל רַבִּי אֱלִיעֶזֶר, וַעֲדַיִן מַטִּין וְעוֹמְדִין.
Rabbi Eliezer then said to them: If the halakha is in accordance with my opinion, the walls of the study hall will prove it. The walls of the study hall leaned inward and began to fall. Rabbi Yehoshua scolded the walls and said to them: If Torah scholars are contending with each other in matters of halakha, what is the nature of your involvement in this dispute? The Gemara relates: The walls did not fall because of the deference due Rabbi Yehoshua, but they did not straighten because of the deference due Rabbi Eliezer, and they still remain leaning.
חָזַר וְאָמַר לָהֶם: אִם הֲלָכָה כְּמוֹתִי – מִן הַשָּׁמַיִם יוֹכִיחוּ. יָצָאתָה בַּת קוֹל וְאָמְרָה: מָה לָכֶם אֵצֶל רַבִּי אֱלִיעֶזֶר, שֶׁהֲלָכָה כְּמוֹתוֹ בְּכׇל מָקוֹם.
Rabbi Eliezer then said to them: If the halakha is in accordance with my opinion, Heaven will prove it. A Divine Voice emerged from Heaven and said: Why are you differing with Rabbi Eliezer, as the halakha is in accordance with his opinion in every place that he expresses an opinion?
עָמַד רַבִּי יְהוֹשֻׁעַ עַל רַגְלָיו וְאָמַר: ״לֹא בַשָּׁמַיִם הִיא!״ מַאי ״לֹא בַּשָּׁמַיִם הִיא״? אָמַר רַבִּי יִרְמְיָה: שֶׁכְּבָר נִתְּנָה תּוֹרָה מֵהַר סִינַי, אֵין אָנוּ מַשְׁגִּיחִין בְּבַת קוֹל, שֶׁכְּבָר כָּתַבְתָּ בְּהַר סִינַי בַּתּוֹרָה ״אַחֲרֵי רַבִּים לְהַטֹּת״. אַשְׁכְּחֵיהּ רַבִּי נָתָן לְאֵלִיָּהוּ, אֲמַר לֵיהּ: מַאי עָבֵיד קוּדְשָׁא בְּרִיךְ הוּא בְּהַהִיא שַׁעְתָּא? אֲמַר לֵיהּ: קָא חָיֵיךְ וְאָמַר, ״נִצְּחוּנִי בָּנַי! נִצְּחוּנִי בָּנַי!״
Rabbi Yehoshua stood on his feet and said: It is written: “It is not in heaven” (Deuteronomy 30:12). The Gemara asks: What is the relevance of the phrase “It is not in heaven” in this context? Rabbi Yirmeya says: Since the Torah was already given at Mount Sinai, we do not regard a Divine Voice, as You already wrote at Mount Sinai, in the Torah: “After a majority to incline” (Exodus 23:2). Since the majority of Rabbis disagreed with Rabbi Eliezer’s opinion, the halakha is not ruled in accordance with his opinion. The Gemara relates: Years after, Rabbi Natan encountered Elijah the prophet and said to him: What did the Holy One, Blessed be He, do at that time, when Rabbi Yehoshua issued his declaration? Elijah said to him: The Holy One, Blessed be He, smiled and said: My children have triumphed over Me; My children have triumphed over Me.
Aftermath
Talmud, Seder Nezikin, Bava Metzia 59b:6-11 (Same Sugya):
אָמְרוּ: אוֹתוֹ הַיּוֹם הֵבִיאוּ כׇּל טְהָרוֹת שֶׁטִּיהֵר רַבִּי אֱלִיעֶזֶר וּשְׂרָפוּם בָּאֵשׁ. וְנִמְנוּ עָלָיו וּבֵרְכוּהוּ, וְאָמְרוּ: מִי יֵלֵךְ וְיוֹדִיעוֹ? אֲמַר לָהֶם רַבִּי עֲקִיבָא: אֲנִי אֵלֵךְ, שֶׁמָּא יֵלֵךְ אָדָם שֶׁאֵינוֹ הָגוּן וְיוֹדִיעוֹ, וְנִמְצָא מַחְרִיב אֶת כָּל הָעוֹלָם כּוּלּוֹ.
The Sages said: On that day, the Sages brought all the ritually pure items deemed pure by the ruling of Rabbi Eliezer with regard to the oven and burned them in fire, and the Sages reached a consensus in his regard and ostracized him. And the Sages said: Who will go and inform him of his ostracism? Rabbi Akiva, his beloved disciple, said to them: I will go, lest an unseemly person go and inform him in a callous and offensive manner, and he would thereby destroy the entire world.
מָה עָשָׂה רַבִּי עֲקִיבָא? לָבַשׁ שְׁחוֹרִים וְנִתְעַטֵּף שְׁחוֹרִים, וְיָשַׁב לְפָנָיו בְּרִיחוּק אַרְבַּע אַמּוֹת. אָמַר לוֹ רַבִּי אֱלִיעֶזֶר: עֲקִיבָא, מָה יוֹם מִיָּמִים? אָמַר לוֹ: רַבִּי, כִּמְדוּמֶּה לִי שֶׁחֲבֵירִים בְּדֵילִים מִמֶּךָ. אַף הוּא קָרַע בְּגָדָיו וְחָלַץ מִנְעָלָיו, וְנִשְׁמַט וְיָשַׁב עַל גַּבֵּי קַרְקַע.
What did Rabbi Akiva do? He wore black and wrapped himself in black, as an expression of mourning and pain, and sat before Rabbi Eliezer at a distance of four cubits, which is the distance that one must maintain from an ostracized individual. Rabbi Eliezer said to him: Akiva, what is different about today from other days, that you comport yourself in this manner? Rabbi Akiva said to him: My teacher, it appears to me that your colleagues are distancing themselves from you. He employed euphemism, as actually they distanced Rabbi Eliezer from them. Rabbi Eliezer too, rent his garments and removed his shoes, as is the custom of an ostracized person, and he dropped from his seat and sat upon the ground.
זָלְגוּ עֵינָיו דְּמָעוֹת, לָקָה הָעוֹלָם: שְׁלִישׁ בְּזֵיתִים, וּשְׁלִישׁ בְּחִטִּים, וּשְׁלִישׁ בִּשְׂעוֹרִים. וְיֵשׁ אוֹמְרִים: אַף בָּצֵק שֶׁבִּידֵי אִשָּׁה טָפַח. תָּנָא: אַף גָּדוֹל הָיָה בְּאוֹתוֹ הַיּוֹם, שֶׁבְּכָל מָקוֹם שֶׁנָּתַן בּוֹ עֵינָיו רַבִּי אֱלִיעֶזֶר – נִשְׂרַף.
The Gemara relates: His eyes shed tears, and as a result the entire world was afflicted: One-third of its olives were afflicted, and one-third of its wheat, and one-third of its barley. And some say that even dough kneaded in a woman’s hands spoiled. The Sages taught: There was great anger on that day, as any place that Rabbi Eliezer fixed his gaze was burned.
וְאַף רַבָּן גַּמְלִיאֵל הָיָה בָּא בִּסְפִינָה. עָמַד עָלָיו נַחְשׁוֹל לְטַבְּעוֹ. אָמַר: כִּמְדוּמֶּה לִי שֶׁאֵין זֶה אֶלָּא בִּשְׁבִיל רַבִּי אֱלִיעֶזֶר בֶּן הוּרְקָנוֹס. עָמַד עַל רַגְלָיו וְאָמַר: רִבּוֹנוֹ שֶׁל עוֹלָם, גָּלוּי וְיָדוּעַ לְפָנֶיךָ שֶׁלֹּא לִכְבוֹדִי עָשִׂיתִי, וְלֹא לִכְבוֹד בֵּית אַבָּא עָשִׂיתִי, אֶלָּא לִכְבוֹדְךָ, שֶׁלֹּא יִרְבּוּ מַחְלוֹקוֹת בְּיִשְׂרָאֵל. נָח הַיָּם מִזַּעְפּוֹ.
And even Rabban Gamliel, the Nasi of the Sanhedrin at Yavne, the head of the Sages who were responsible for the decision to ostracize Rabbi Eliezer, was coming on a boat at the time, and a large wave swelled over him and threatened to drown him. Rabban Gamliel said: It seems to me that this is only for the sake of Rabbi Eliezer ben Hyrcanus, as God punishes those who mistreat others. Rabban Gamliel stood on his feet and said: Master of the Universe, it is revealed and known before You that neither was it for my honor that I acted when ostracizing him, nor was it for the honor of the house of my father that I acted; rather, it was for Your honor, so that disputes will not proliferate in Israel. In response, the sea calmed from its raging.
אִימָּא שָׁלוֹם, דְּבֵיתְהוּ דְּרַבִּי אֱלִיעֶזֶר, אֲחָתֵיהּ דְּרַבָּן גַּמְלִיאֵל הֲוַאי. מֵהָהוּא מַעֲשֶׂה וְאֵילָךְ, לָא הֲוָה שָׁבְקָה לֵיהּ לְרַבִּי אֱלִיעֶזֶר לְמִיפַּל עַל אַפֵּיהּ. הָהוּא יוֹמָא רֵישׁ יַרְחָא הֲוָה, וְאִיחַלַּף לַהּ בֵּין מָלֵא לְחָסֵר. אִיכָּא דְּאָמְרִי: אֲתָא עַנְיָא וְקָאֵי אַבָּבָא, אַפִּיקָא לֵיהּ רִיפְתָּא.
The Gemara further relates: Imma Shalom, the wife of Rabbi Eliezer, was the sister of Rabban Gamliel. From that incident forward, she would not allow Rabbi Eliezer to lower his head and recite the taḥanun prayer, which includes supplication and entreaties. She feared that were her husband to bemoan his fate and pray at that moment, her brother would be punished. A certain day was around the day of the New Moon, and she inadvertently substituted a full thirty-day month for a deficient twenty-nine-day month, i.e., she thought that it was the New Moon, when one does not lower his head in supplication, but it was not. Some say that a pauper came and stood at the door, and she took bread out to him. The result was that she left her husband momentarily unsupervised.
אַשְׁכַּחְתֵּיהּ דְּנָפֵל עַל אַנְפֵּיהּ. אֲמַרָה לֵיהּ: קוּם, (קטלית לאחי) [קְטַלְיתֵּהּ לְאָח]. אַדְּהָכִי נְפַק שִׁיפּוּרָא מִבֵּית רַבָּן גַּמְלִיאֵל דִּשְׁכֵיב. אֲמַר לַהּ: מְנָא יְדַעְתְּ? אֲמַרָה לֵיהּ: כָּךְ מְקוּבְּלַנִי מִבֵּית אֲבִי אַבָּא: כׇּל הַשְּׁעָרִים נִנְעָלִים חוּץ מִשַּׁעֲרֵי אוֹנָאָה.
When she returned, she found him and saw that he had lowered his head in prayer. She said to him: Arise, you already killed my brother. Meanwhile, the sound of a shofar emerged from the house of Rabban Gamliel to announce that the Nasi had died. Rabbi Eliezer said to her: From where did you know that your brother would die? She said to him: This is the tradition that I received from the house of the father of my father: All the gates of Heaven are apt to be locked, except for the gates of prayer for victims of verbal mistreatment.
Case 2: Spirit of Nabouth lying to Prophets of Ahab
Nabouth was executed by King Ahab based on false charges set up by his wife Jezebel.
In 1 Kings 22:22-23 and 2 Chronicles 18:21-22, Micaiah said that a lying spirit misinformed the other Prophets gathered by Ahab to tell him that he’ll win the war.
It was the Spirit of Nabout who misled him, and his desire to deceive Ahab also cost him his relationship with G-d, as he said “Go out” and do so:
Talmud, Seder Nezikin, Sanhedrin, Page 89a, Lines 15-16:
הַמִּתְנַבֵּא מַה שֶּׁלֹּא שָׁמַע, כְּגוֹן צִדְקִיָּה בֶן כְּנַעֲנָה, דִּכְתִיב: ״וַיַּעַשׂ לוֹ צִדְקִיָּהוּ בֶן כְּנַעֲנָה קַרְנֵי בַרְזֶל״. מַאי הֲוָה לֵיהּ לְמֶעְבַּד? רוּחַ נָבוֹת אַטְעִיתֵיהּ, דִּכְתִיב: ״וַיֹּאמֶר ה׳ מִי יְפַתֶּה אֶת אַחְאָב וְיַעַל וְיִפֹּל בְּרָמֹת גִּלְעָד״. ״וַיֵּצֵא הָרוּחַ וַיַּעֲמֹד לִפְנֵי ה׳ וַיֹּאמֶר אֲנִי אֲפַתֶּנּוּ״. ״וַיֹּאמֶר תְּפַתֶּה וְגַם תּוּכָל צֵא וַעֲשֵׂה כֵן״.
§ The mishna lists among those liable to be executed as a false prophet one who prophesies that which he did not hear. The Gemara comments: For example, Zedekiah, son of Chenaanah, who prophesied that Ahab should wage war against the kingdom of Aram and would be successful, as it is written: “And Zedekiah, son of Chenaanah, made for him horns of iron, and said: So says the Lord: With these shall you gore the Arameans, until they are consumed” (II Chronicles 18:10). The Gemara asks: What was Zedekiah, son of Chenaanah, to do? Why was he held responsible? After all, the spirit of Naboth misled him, as it is written: “And the Lord said: Who shall entice Ahab, that he may go up and fall at Ramoth Gilead…and there came forth the spirit, and stood before the Lord, and said: I will entice him. And He said…You shall entice him, and shall prevail also; go forth, and do so” (I Kings 22:20–22).
אָמַר רַב יְהוּדָה: מַאי ״צֵא״? צֵא מִמְּחִיצָתִי. מַאי רוּחַ? אָמַר רַבִּי יוֹחָנָן: רוּחוֹ שֶׁל נָבוֹת הַיִּזְרְעֵאלִי.
Rav Yehuda says: What is the meaning of the apparently unnecessary term: “Go forth”? Could the Lord not have sufficed with telling the spirit: Do so? The term means: Go forth from My proximity. Since the spirit volunteered to engage in deceit, it was no longer fit to stand in proximity to God. What is the identity of the spirit that undertook to entice Ahab? Rabbi Yoḥanan says: It was the spirit of Naboth the Jezreelite, who was eager to take revenge upon Ahab.
Talmud, Seder Nezikin, Bava Metzia, Page 102b, Lines 15-17:
״וַיֵּצֵא הָרוּחַ וַיַּעֲמֹד לִפְנֵי ה׳ וַיֹּאמֶר אֲנִי אֲפַתֶּנּוּ וַיֹּאמֶר ה׳ אֵלָיו בַּמָּה. וַיֹּאמֶר אֵצֵא וְהָיִיתִי רוּחַ שֶׁקֶר בְּפִי כׇּל נְבִיאָיו וַיֹּאמֶר תְּפַתֶּה וְגַם תּוּכָל צֵא וַעֲשֵׂה כֵן״. מַאי רוּחַ? אָמַר רַבִּי יוֹחָנָן: רוּחוֹ שֶׁל נָבוֹת הַיִּזְרְעֵאלִי.
With regard to God’s search for a volunteer to entice Ahab, it is written: “And there came forth a spirit, and stood before the Lord, and said: I will entice him. And the Lord said to him: With what? And he said: I will go out, and I will be a lying spirit in the mouth of all his prophets. And He said: You shall entice him, and also prevail; go out, and do so” (I Kings 22:21–22). The Gemara asks: What spirit was that? Rabbi Yoḥanan says: It was the spirit of Naboth the Jezreelite, who sought to take revenge against Ahab.
מַאי ״צֵא״? אָמַר רָבִינָא: ״צֵא מִמְּחִיצָתִי״, שֶׁכֵּן כְּתִיב: ״דֹּבֵר שְׁקָרִים לֹא יִכּוֹן לְנֶגֶד עֵינָי״. אָמַר רַב פָּפָּא: הַיְינוּ דְּאָמְרִי אִינָשֵׁי: דְּפָרַע קִינֵּיהּ מַחְרֵיב בֵּיתֵיהּ.
The Gemara asks: What is the meaning of the term “go out” that God instructed the spirit? Ravina says that it means: Go out from within my partition and do not return, as so it is written: “He that speaks falsehoods shall not be established before My eyes” (Psalms 101:7). The spirit that volunteered to lie may no longer be before God. Rav Pappa says that this is in accordance with the adage that people say: One who avenges due to his zealotry destroys his own house. The spirit of Naboth that sought revenge was expelled from before God.
״וַיַּעַשׂ אַחְאָב אֶת הָאֲשֵׁרָה וַיּוֹסֶף אַחְאָב לַעֲשׂוֹת לְהַכְעִיס אֶת ה׳ אֱלֹהֵי יִשְׂרָאֵל מִכֹּל מַלְכֵי יִשְׂרָאֵל אֲשֶׁר הָיוּ לְפָנָיו״. אָמַר רַבִּי יוֹחָנָן: שֶׁכָּתַב עַל דַּלְתוֹת שֹׁמְרוֹן אַחְאָב כָּפַר בֵּאלֹהֵי יִשְׂרָאֵל, לְפִיכָךְ אֵין לוֹ חֵלֶק בֵּאלֹהֵי יִשְׂרָאֵל.
With regard to the verse: “And Ahab made the ashera; and Ahab did more to provoke the Lord, God of Israel, to anger than all the kings of Israel that were before him” (I Kings 16:33), Rabbi Yoḥanan says: It means that he wrote on the doors of Samaria: Ahab denies the existence of the God of Israel, therefore he has no share in the God of Israel.
Jehoshaphat knew two Prophets won’t speak in the same manner. That’s why he asked for another Prophet of G-d.
Talmud, Seder Nezikin, Sanhedrin 89a, 17-19:
הֲוָה לֵיהּ לְמֵידַק, כִּדְרַבִּי יִצְחָק, דְּאָמַר רַבִּי יִצְחָק: סִיגְנוֹן אֶחָד עוֹלֶה לְכַמָּה נְבִיאִים, וְאֵין שְׁנֵי נְבִיאִים מִתְנַבְּאִין בְּסִיגְנוֹן אֶחָד.
The Gemara comments: It was incumbent upon Zedekiah to discern between actual prophecies and false prophecies, in accordance with the statement of Rabbi Yitzḥak, as Rabbi Yitzḥak says: A prophetic vision relating to one and the same subject matter [sigenon] may appear to several prophets, but two prophets do not prophesy employing one and the same style of expression.
עוֹבַדְיָה אָמַר: ״זְדוֹן לִבְּךָ הִשִּׁיאֶךָ״. יִרְמְיָה אָמַר: ״תִּפְלַצְתְּךָ הִשִּׁיא אֹתָךְ זְדוֹן לִבֶּךָ״. וְהָנֵי, מִדְּקָאָמְרִי כּוּלְּהוּ כַּהֲדָדֵי, שְׁמַע מִינַּהּ לָא כְּלוּם קָאָמְרִי.
An example of identical content expressed in different styles is the prophecy where Obadiah said: “The pride of your heart has beguiled you” (Obadiah 1:3), and Jeremiah said a similar message employing slightly different language: “Your terribleness has deceived you, even the pride of your heart” (Jeremiah 49:16). And with regard to these false prophets, from the fact that in their case all the prophets are saying their prophecies like each other, i.e., employing an identical style, conclude from it that they are saying nothing of substance and that it is a false prophecy.
דִּילְמָא לָא הֲוָה יָדַע לֵיהּ לְהָא דְּרַבִּי יִצְחָק? יְהוֹשָׁפָט הֲוָה הָתָם, וְקָאָמַר לְהוּ. דִּכְתִיב: ״וַיֹּאמֶר יְהוֹשָׁפָט הַאֵין פֹּה נָבִיא עוֹד לַה׳״. אֲמַר לֵיהּ: הָא אִיכָּא כֹּל הָנֵי. אֲמַר לֵיהּ: כָּךְ מְקוּבְּלַנִי מִבֵּית אֲבִי אַבָּא: סִיגְנוֹן אֶחָד עוֹלֶה לְכַמָּה נְבִיאִים, וְאֵין שְׁנֵי נְבִיאִים מִתְנַבְּאִים בְּסִיגְנוֹן אֶחָד.
The Gemara asks: Perhaps Zedekiah, son of Chenaanah, did not know this statement of Rabbi Yitzḥak, and therefore should not have been held responsible? The Gemara answers: Jehoshaphat, king of Judea, was there, and he said to them that they were false prophets, as it is written: “But Jehoshaphat said: Is there not here besides a prophet of the Lord, that we might inquire of him?” (I Kings 22:7). Ahab said to him: But aren’t there all these prophets here? Why seek others? Jehoshaphat said to Ahab: This is the tradition that I received from the house of the father of my father, the house of David: A prophetic vision relating to one and the same subject matter may appear to several prophets, but two prophets do not prophesy employing one and the same style of expression. They are false prophets, as they employed the same language when stating their prophecy.
Case 3: 13 Principles of Faith by Maimonaides
See The Sixth Principle: Prophecy, Lesson 19: Various Prophetic Terms and Experiences (Principles VI and VII, Pages 200-215), here: https://books.google.co.cr/books?id=t4TclPFTzfQC&lpg=PP1&pg=PA199#v=onepage&q&f=false
Urim and Thummim are metioned in Exodus 28:31. See more on them at Pages 289-302 (Principles VI and VII, Appendix B: The Priestly Oracle), here: https://books.google.co.cr/books?id=t4TclPFTzfQC&lpg=PP1&pg=PA289#v=onepage&q&f=false