Leviticus 21:21-23
21:21 No man from the descendants of Aaron the priest who has a physical flaw may step forward to present the Lord’s gifts; he has a physical flaw, so he must not step forward to present the food of his God.
21:22 He may eat both the most holy and the holy food of his God,
21:23 but he must not go into the veil-canopy or step forward to the altar because he has a physical flaw. Thus he must not profane my holy places, for I am the Lord who sanctifies them.’”
Numbers 3:10
3:10 So you are to appoint Aaron and his sons, and they will be responsible for their priesthood; but the unauthorized person who comes near must be put to death.”
Numbers 16:40
16:40 It was a memorial for the Israelites, that no outsider who is not a descendant of Aaron should approach to burn incense before the Lord, that he might not become like Korah and his company – just as the Lord had spoken by the authority of Moses.
Numbers 18:7
18:7 But you and your sons with you are responsible for your priestly duties, for everything at the altar and within the curtain. And you must serve. I give you the priesthood as a gift for service; but the unauthorized person who approaches must be put to death.”