Rashi on Deuteronomy 18:20/רש"י על דברים י״ח:כ׳

1אשר לא צויתיו לדבר [BUT THE PROPHET WHO SHALL PRESUME TO SPEAK A WORD …] WHICH I HAVE NOT COMMANDED HIM TO SPEAK — but which I may have commanded his fellow-prophet to speak (Sanhedrin 89a),

2ואשר ידבר בשם אלהים אחרים OR THAT SHALL SPEAK IN THE NAME OF OTHER GODS — even though he be in exact agreement with the Halacha, — that he forbids what is forbidden, or permits what is permitted, [but does so in the name of other gods] (Sanhedrin 89a),

3ומת HE SHALL DIE — by strangulation. — There are three false prophets who must be put to death by man (by the court): one who prophesies something which he has not heard (was not commanded to prophesy); one who prophesies something that was not spoken unto him but was spoken unto his fellow-prophet, and finally one who prophesies in the name of an idol. But he who suppresses his prophecy (does not utter it as commanded to do), and he who transgresses the words of a prophet, or, being himself a prophet, transgresses his own words — their death is by the Heavenly Judge, for it is said, (v. 19) “I” will require it from him” (Sanhedrin 89a).