Numbers 18:6-8

I myself have selected your fellow Levites from among the Israelites as a gift to you, dedicated to the LORD to do the work at the tent of meeting. But only you and your sons may serve as priests in connection with everything at the altar and inside the curtain. I am giving you the service of the priesthood as a gift. Anyone else who comes near the sanctuary is to be put to death.” Then the LORD said to Aaron, “I myself have put you in charge of the offerings presented to me; all the holy offerings the Israelites give me I give to you and your sons as your portion, your perpetual share.