Numbers 18:6-8

I myself chose your fellow Levites from among the Israelites as a gift given for you to the LORD, to work at the Meeting Tent. But only you and your sons may serve as priests. Only you may serve at the altar or go behind the curtain. I am giving you this gift of serving as a priest, and anyone else who comes near the Holy Place will be put to death.” Then the LORD said to Aaron, “I myself make you responsible for the offerings given to me. All the holy offerings that the Israelites give to me, I give to you and your sons as your share, your continual portion.