The 27th prohibition is that we are forbidden from prophesizing falsely, either by prophesizing in G‑d's Name even though G‑d has not spoken,1 or by prophesizing words which G‑d (exalted be He) has spoken to someone other than himself, and he attributes it to himself, saying that G‑d has spoken to him, when in reality, He has not.

The source of this commandment is G‑d's statement,2 "If a prophet intentionally makes a declaration in My name when I have not commanded him to speak."

One who transgresses this commandment is also3 executed by strangulation — when listing those who are punished by strangulation, our Sages4 included a false prophet. There they stated, "Three are executed by man5: one who 'intentionally makes a declaration in My name,' i.e. when he has not heard; one who speaks 'when I have not commanded him to speak,' but I have commanded someone else to speak, i.e. one who prophesizes when he himself has not been spoken to; and one who6 'speaks in the name of other gods,' i.e. in the name of an idol." Regarding all of them it is written,7 "That prophet shall die," and when the type of execution is not specified, it refers to strangulation.8

The details of this mitzvah have been explained in the eleventh chapter of Sanhedrin.9