Negative commandment · Lo Ta’aseh #27 of 613
Not to relate false prophecies
שלא להתנבא בשקר
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 relate false prophecies, as [Deuteronomy 18:20] states: "When a prophet presumptuously makes a declaration in My name which I have not commanded him...."
שלא להתנבא בשקר, שנאמר "אשר יזיד לדבר דבר בשמי, את אשר לא ציוויתיו" (דברים יח,כ).
Why · Sefer HaChinuch #517
To not prophecy falsely
“It is from the roots of the commandment [that it is] since there would be great destruction and much evil to our sacred and perfect religion through this.”
Who & when
“And this prohibition 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