And the 108th prohibition is that we are forbidden from redeeming a kosher firstborn animal.

The source of this commandment is G‑d's statement1 (exalted be He), "You may not redeem, however, the firstborn of an ox, sheep, or goat since they are sacred."

One is, however, allowed to sell it, as explained in tractate Bechoros.2 The Sifra3 says, "Regarding a firstborn animal it is written, 'you may not redeem,' but it may be sold."

[The details4 of this mitzvah are explained in tractate Bechoros5 and in tractate Ma'aser Sheini.6]