Numbers 18:6-7

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.”

Numbers 18:29

18:29 From all your gifts you must offer up every raised offering due the Lord, from all the best of it, and the holiest part of it.’