And the 109th prohibition is that we are forbidden from selling a tithed animal1 in any way.2

The source of this commandment is G‑d's statement,3 "Do not redeem it."

The Sifra says, "Regarding the tithed animal it is written, 'do not redeem it.' It may not be sold, whether live or slaughtered, whether unblemished or blemished."

The details of this mitzvah as well as the preceding one4 are explained in tractate Bechoros5 and in the beginning of tractate Ma'aser Sheini.6