Numbers 18:5-7

18:5 You will be responsible for the care of the sanctuary and the care of the altar, so that there will be no more wrath on the Israelites. 18:6 I myself have chosen your brothers the Levites from among the Israelites. They are given to you as a gift from the Lord, to perform the duties of the tent of meeting. 18:7 But you and your sons with you are responsible for your priestly duties, for everything at the altar and within the curtain. And you must serve. I give you the priesthood as a gift for service; but the unauthorized person who approaches must be put to death.”