Numbers 18:5-7

5And you shall attend to the duties of the sanctuary and the duties of the altar, that there may be no more wrath on the children of Israel.

6Behold, I Myself have taken your brethren the Levites from among the children of Israel; they are a gift to you, given by the Lord, to do the work of the tabernacle of meeting.

7Therefore you and your sons with you shall attend to your priesthood for everything at the altar and behind the veil; and you shall serve. I give your priesthood to you as a gift for service, but the outsider who comes near shall be put to death.”