הלכה אויף במדבר 18:7

That one, not a descendant of Aaron in the male line, shall not serve (in the Sanctuary), as it is said, “And a stranger shall not come nigh unto you” (Num. 18:4-7).

And behold, Ramban, may his memory be blessed, (on Sefer HaMitzvot, Shorashim 12) does not calculate the law of the sin-offering, the guilt-offering, the burnt-offering and the peace-offerings in the tally of the commandments, based on the fundamental principle that I have said. As the main commandment comes about the obligation of the bringing of the sacrifice. But it is not right to calculate the order of its processing afterwards as a commandment on its own. As perforce Scripture had to teach us the process of each one of them, once we became obligated about them. And this is his language: “Every process of the sacrifices is one commandment and all of the seed of Aharon were commanded in its service, as it is stated (Numbers 18:7), ‘and you shall serve, the service of the gift that I gave your priesthood.’ And its explanation is that they shall serve all of the service of the priesthood, since it is a service and a gift of Mine and a gift to you, that you get reward for it when you take from the table of the Higher realm, may He be blessed.” To here [are his words]. And with all of this, we shall not veer from the path of the tally of Rambam, may his memory be blessed, which (we have taken hold of) [permeates] the commandments. And the thing that is difficult, the difficulty should be attributed to us and not to him — as he is truthfully our source in this endeavor, and from his hand did we merit it. The righteous man should rest in his resting place.