Halakhah su Deuteronomio 18:20

To not prophecy falsely: That we were prevented from not prophesying falsely, meaning to say that no person should say that words were said to him in prophecy from God, may He be blessed, when God, may He be blessed, did not say them. And likewise included in this negative commandment is even if he says things that were said in prophecy to someone else, and he falsely says that he was commanded to say them. And about this is it stated (Deuteronomy 18:20), "But the prophet that wantonly speaks something in My name; that which I did not command him to speak, etc." And so [too,] did they, may their memory be blessed, say in Sanhedrin 89a, "'That wantonly speaks something in My name' - that is the one who prophesies what he did not hear; "that which I did not command him' - him did I not command it, but to his fellow, I did command it - that is the one that prophesies what was not said to him, but it was said to his fellow."

And this prohibition is practiced in every place and at all times by males and females. And one who transgresses it and prophesies falsely - meaning to say that he said things in the name of God that God did not say; and likewise, one who prophesies that which God said to his fellow and not to him - is liable for the death penalty. And his death is by strangulation, as it is stated about this (Deuteronomy 18:20), "and that prophet shall die" - and they, may their memory be blessed, said that any death penalty stated undifferentiated in the Torah is only strangulation.

To not prophesy in the name of idolatry: To not prophesy in the name of idolatry - for example, he says that idolatry x commanded to worship it, and promises a reward for its worshipers and affrights the one who does not serve it with a punishment; as the prophets of Baal and Asherah would say, and like is mentioned in the books of the Prophets. And likewise included in this is if he says that God commanded to worship idolatry x. And the Scripture does not come about this with a specific clear warning, however the punishment of the one who prophesies in the name of idolatry is clear in the Scriptures - that he is liable for death, as it is stated about this (Deuteronomy 18:20), "and the one who speaks in the name of other gods, that prophet shall die." And this death is strangulation, as we wrote adjacently. And we have already known the principle that they, may their memory be blessed, taught us, "He does not punish unless He warned." And hence we shall say that the warning of this matter was included in "and the name of other gods shall you not mention" (Exodus 23:13), that we wrote in Parshat Mishpatim as a negative commandment of it own about another matter (Sefer HaChinukh 86). And it is not impossible for one negative commandment to prevent several things - and its law is not like the law of a general negative commandment, since the punishment of each and every matter is elucidated. This is the opinion of Rambam, may his memory be blessed (in the Sefer HaMitzvot LaRambam, Shoresh 14 at the end).