הלכה על ויקרא 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 commadmeent; '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.