Numbers 18:5-7

5So you are to attend to the Sanctuary and the care of the altar, so no further wrath will fall on Bnei-Yisrael.

6“See, I have personally taken your kinsmen, the Levites, from among Bnei-Yisrael as a gift for you and your sons with you, dedicated to Adonai to serve in the work of the Tent of Meeting.

7But you and your sons with you are to maintain your priesthood for everything pertaining to the altar and inside the parokhet . I am giving you the ministry of the priesthood as a gift. Anyone unauthorized who approaches will die.”