Negative commandment · Lo Ta’aseh #29 of 613
Not to refrain from executing a false prophet, nor to fear him
שלא נימנע מהריגת נביא השקר ולא נירא ממנו
The verse · Devarim 18:22
אֲשֶׁר יְדַבֵּר הַנָּבִיא בְּשֵׁם ה׳ וְלֹא־יִהְיֶה הַדָּבָר וְלֹא יָבֹא הוּא הַדָּבָר אֲשֶׁר לֹא־דִבְּרוֹ ה׳ בְּזָדוֹן דִּבְּרוֹ הַנָּבִיא לֹא תָגוּר מִמֶּנּוּ
“Know that when a prophet speaks in the name of the Lord, if the thing follow not, nor come to pass, that is the thing which the Lord has not spoken, but the prophet has spoken it out of presumption; thou shalt not be afraid of him.”
The Rambam’s words
Not to refrain from executing a false prophet, nor to fear him, as [Deuteronomy 18:22] states: "Do not fear him."
שלא נימנע מהריגת נביא השקר ולא נירא ממנו, שנאמר "לא תגור ממנו" (דברים יח,כב).
Why · Sefer HaChinuch #519
That we not fear from the killing of a false prophet
“From the roots of the commandment are that which I have written adjacently about one who prophesies falsely.”
Who & when
“This commandment is practiced at the time of the [Temple] when Israel are in their inhabitation; since [only] then is it in our hand to judge capital cases.”
Where in the Mishneh Torah
Hilchot Foreign Worship and Customs of the Nations הלכות עבודה זרה וחוקות הגויים