Avodas Kochavim 5:6
“A prophet who prophesizes in the name of idols” is one who says that a certain idol or star told him that we are commanded to do or not to do a certain thing. This is so even if he states the law accurately, such as by declaring something impure to be impure and something pure to be pure.
If a false prophet was given warning beforehand, he is liable to death by strangulation. Deuteronomy 18:20 says, “The prophet who speaks in the name of other gods shall die.” The prohibition against this appears in Exodus 23:13, “Do not mention the name of other gods.”
Avodas Kochavim 5:7
We are not allowed to engage an idolatrous prophet in discussion or debate. We may not ask him to give us a sign or to perform a wonder; if he does so on his own, we may not pay any attention to it. Whoever considers such a person’s tricks, thinking they might be a sign that he is true, violates the prohibition of “Do not listen to the words of that prophet” (Deuteronomy 13:4).
A false prophet is executed by strangulation. He is executed even though he speaks in the name of God and doesn’t add to or subtract from the mitzvos, as per Deuteronomy 18:20, “The prophet who dares to speak in My name that which I did not command him shall die.”