הלכה אויף ויקרא 6:9

That the remainder of the meal-offerings shall be eaten, as it is said, “And the remainder thereof shall Aaron and his sons eat” (Lev. 6:9).

The commandment of eating the remainders of the meal-offering: That the priests were commanded to eat the remainders of the meal-offerings — meaning to say after they separated from it that which they would offer on the altar, they would eat all the rest, as it is stated (Leviticus 6:9), “What is left of it shall be eaten by Aharon and his sons; as matsahs shall it be eaten, etc.” And the language of Sifra, Tzav 2:9 is, “‘As matsahs shall it be eaten’ is a commandment; ‘her levirate husband shall have sexual relations with her’ (Deuteronomy 25:5) is a commandment” — meaning to say that both of them are positive commandments, not optional.