Numbers 18:5-7

Amplified Bible

⁵ So 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].^{(A) 6} Behold, 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). ⁷ Therefore 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.”^(B)