Numbers 18:5-7

So you shall perform the duties of the sanctuary and the duties of the altar, so that there will no longer be wrath on the sons of Israel. 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 that concerns the altar and inside the veil, and you are to perform service. I am giving you the priesthood as a service that is a gift, and the unauthorized person who comes near shall be put to death.”