Numbers 18:6-8

Behold, I Myself have taken your fellow Levites from among the sons of Israel; they are a gift to you, dedicated to the LORD, to perform the service for the tent of meeting. But you and your sons with you shall attend to your priesthood for everything concerning the altar and inside the veil, and you are to perform service. I am giving you the priesthood as a bestowed service, but the outsider who comes near shall be put to death.” Then the LORD spoke to Aaron, “Now behold, I Myself have given you charge of My offerings, even all the holy gifts of the sons of Israel I have given them to you as a portion and to your sons as a perpetual allotment.