Numbers 18:3, 6-7

But as the Levites go about all their assigned duties at the Tabernacle, they must be careful not to go near any of the sacred objects or the altar. If they do, both you and they will die.

I myself have chosen your fellow Levites from among the Israelites to be your special assistants. They are a gift to you, dedicated to the LORD for service in the Tabernacle. But you and your sons, the priests, must personally handle all the priestly rituals associated with the altar and with everything behind the inner curtain. I am giving you the priesthood as your special privilege of service. Any unauthorized person who comes too near the sanctuary will be put to death.”