Numbers 18:7-8

7Therefore you and your sons with you shall attend to your priesthood for everything concerning the altar [of burnt offering and the altar of incense] and [of the Holy of Holies] within the veil, and you are to serve. I am giving you the priesthood as a bestowed service, but the stranger (outsider, layman) who approaches shall be put to death.”

8Then the Lord spoke to Aaron, “Now behold, I [the Lord] have entrusted you with My heave offerings, even all the holy gifts of the Israelites, I have given them to you as a portion, and to your sons as a continual allotment.