And the 88th mitzvah is that kohanim are commanded to eat the leftover parts of the meal-offerings.

The source of this commandment is G‑d's statement1 (exalted be He), "Aaron and his descendants shall then eat the rest of [the meal-offering]. It must be eaten as matzah."

In the words of the Sifra: "The verse 'It must be eaten as matzah' constitutes a positive commandment. This is similar to the verse,2 '[When brothers live together and one of them dies childless...] her husband's brother must cohabit with her,' which also constitutes a positive commandment." This means that eating the leftover portion of the meal-offerings is an obligation and not just a statement giving permission, just like the commandment of cohabiting with a yevamah.3

The details of this mitzvah are explained in the relevant place — tractate Menachos.4

Scripture itself states that this commandment is exclusively for male [kohanim], as seen from the verse,5 "Every male among Aaron's descendants may eat it."