Numbers 18:3, 6-7

They are to be responsible to you and are to perform all the duties of the tent, but they must not go near the furnishings of the sanctuary or the altar. Otherwise both they and you will die.

I myself have selected your fellow Levites from among the Israelites as a gift to you, dedicated to the LORD to do the work at the tent of meeting. But only you and your sons may serve as priests in connection with everything at the altar and inside the curtain. I am giving you the service of the priesthood as a gift. Anyone else who comes near the sanctuary is to be put to death.”