Negative commandment · Lo Ta’aseh #26 of 613
Not to prophesy in the name of false gods
שלא להתנבא בשמה
The verse · Devarim 18:20
אַךְ הַנָּבִיא אֲשֶׁר יָזִיד לְדַבֵּר דָּבָר בִּשְׁמִי אֵת אֲשֶׁר לֹא־צִוִּיתִיו לְדַבֵּר וַאֲשֶׁר יְדַבֵּר בְּשֵׁם אֱלֹהִים אֲחֵרִים וּמֵת הַנָּבִיא הַהוּא
“But the prophet, who shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, that prophet shall die.”
The Rambam’s words
Not to prophesy in the name of false gods, as [Deuteronomy 18:20] states: "[The prophet]... who speaks in the name of other gods [shall die]."
שלא להתנבא בשמה, שנאמר "ואשר ידבר, בשם אלוהים אחרים" (דברים יח,כ).
Why · Sefer HaChinuch #518
To not prophesy in the name of idolatry
“The root of the commandment about all idolatry is well-known.”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Foreign Worship and Customs of the Nations הלכות עבודה זרה וחוקות הגויים