Numbers 18:7

Therefore you and your sons with you shall attend to your priesthood for everything of the altar [of burnt offering and the altar of incense] and [of the Holy of Holies] within the veil, and you shall serve. I give you your priesthood as a service of gift. And the stranger [anyone other than Moses or your sons, Aaron] who comes near shall be put to death. [Exod. 40:18, 20, 26.]