Numbers 18:4
18:4 They must join with you, and they will be responsible for the care of the tent of meeting, for all the service of the tent, but no unauthorized person may approach you.
Numbers 18:7
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.”