The 81st mitzvah is that we are commanded to redeem a first-born donkey with a sheep, and to give the sheep to a kohen — unless we give the monetary value.1
The source of this commandment is G‑d's statement,2 "The first-born of a donkey you shall redeem with a sheep."
The details of this mitzvah are also explained in tractate B'choros.
Levites are also exempt from this mitzvah.
