The 28th prohibition is that we are forbidden from listening to the prophecy of one who prophesizes in the name of an idol, i.e. we should not dispute him, nor ask him, "What is your miracle, and what is your proof of this," as we do with someone who prophesizes in the name of G‑d.1 Rather, as soon as we hear him prophesize in its name, we must warn him,2 just as we must warn any transgressor; and if he persists in his claim, we must punish him in the way the Torah prescribes, and pay no attention to his miracles and proofs.

The source of this commandment is G‑d's statement (exalted be He),3 "Do not listen to the words of that prophet."

The details of this mitzvah have been explained in the eleventh chapter of Sanhedrin.4