Sefer HaChinuch ספר החינוך

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.”

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. As the primary truth without limits among people is through the prophets, and [hence] the Torah commanded us to believe them and to follow their correct counsel and their complete intellects. And so when wanton men rise up to say things that God did not command them, they put out slander against prophecy — which is a great principle [of faith] among us, the holy people. And because of them, the heart of all the people will hesitate about true prophets. And there is a great destruction also if a person says what another person was commanded about. As in that this person says that he is prophet and he was commanded about this, and we see that his words are validated like the words of true prophets; we will assume him to be a holy man of God and a messenger of God, and we will believe him. And [so] we will take from his actions as a proof (lesson) for all of our practices. But since his merit and substance are not great [enough] to be the messenger for that prophecy that he said, perhaps he is not fitting to rely upon in everything he says and does. And [so] he will mislead the masses in their following his counsel.

The laws of the commandment are short, and they are in Sanhedrin (the chapter [entitled] HaNechenakin).

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.