The 8th prohibition is that we are forbidden from performing the practice of ov. This1 consists of burning a specific type of incense and performing certain actions. The person who does this then imagines2 that he hears a voice speaking from under his armpit answering his questions. This is a form of idolatry.

The source of this commandment is G‑d's statement,3 "Do not turn to the idols called ov."

In the words of the Sifra, "Ov refers to a Pitom4 who speaks from his armpit." One who transgresses this prohibition intentionally is executed by stoning; if he is not executed by stoning5, he is punished by karet; and if he did so unintentionally, he must bring a sin-offering. This applies to the one who actually performed the actions himself.6

The details of this mitzvah have been explained in the seventh chapter of Sanhedrin.7