Numbers 18:5-7

5And you shall attend to the duties of the sanctuary and of the altar, so that wrath may not fall on the Israelites again. 6Behold, I Myself have selected your fellow Levites from the Israelites as a gift to you, dedicated to the LORD to perform the service for the Tent of Meeting. 7But only you and your sons shall attend to your priesthood for everything concerning the altar and what is inside the veil, and you are to perform that service. I am giving you the work of the priesthood as a gift, but any outsider who comes near the sanctuary must be put to death.”