And the 112th prohibition is that when performing melikah1 on a bird brought as a sin-offering, we are forbidden from completely severing its head [from its body].

The source of this prohibition is G‑d's statement2 (exalted be He), "He shall gouge through its neck from the back without severing [the head from the body]."

If he does sever the head, the offering is invalid.

The details of this mitzvah are explained in the 6th chapter of tractate Zevachim.