Numbers 18:5-7

5“You are to be responsible for the care of the sanctuary and the altar, so that my wrath will not fall on the Israelites again.

6I 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.

7But 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.”