Numbers 18:5-7

5So you shall perform the 1duties of the sanctuary and the duties of the altar, 2so that there will no longer be wrath on the sons of Israel. 6Behold, I Myself 3have taken your 4fellow Levites from among the sons of Israel; they are 5a gift to you, 6dedicated to the Lord, to perform the service for the tent of meeting. 7But you and your sons with you shall 7attend 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 8a 9service that is a gift, and 10the 11unauthorized person who comes near shall be put to death.”