Numbers 18:5-7

5So you shall attend to the duties of the sanctuary and the duties of the altar [of burnt offering and the altar of incense], so that there will no longer be wrath on the Israelites [as with Korah, Dathan, and Abiram].

6Behold, I Myself have taken your fellow Levites from among the sons of Israel; they are a gift to you, given (dedicated) to the Lord, to do the service for the Tent of Meeting (tabernacle).

7Therefore you and your sons with you shall attend to your priesthood for everything concerning the altar [of burnt offering and the altar of incense] and [of the Holy of Holies] within the veil, and you are to serve. I am giving you the priesthood as a bestowed service, but the stranger (outsider, layman) who approaches shall be put to death.”