Numbers 18:3, 6-7

And they shall thus attend to your obligation and the obligation of all the tent, but they shall not come near to the furnishings of the sanctuary and the altar, or both they and you will die.

Behold, I Myself have taken your fellow Levites from among the sons of Israel; they are a gift to you, dedicated to the LORD, to perform the service for the tent of meeting. But you and your sons with you shall attend to your priesthood for everything concerning the altar and inside the veil, and you are to perform service. I am giving you the priesthood as a bestowed service, but the outsider who comes near shall be put to death.”