Numbers 18:3, 6-7

They are under your control, to do all the work that needs to be done in the Tent. But they must not go near the things in the Holy Place or near the altar. If they do, both you and they will die.

I myself chose your fellow Levites from among the Israelites as a gift given for you to the LORD, to work at the Meeting Tent. But only you and your sons may serve as priests. Only you may serve at the altar or go behind the curtain. I am giving you this gift of serving as a priest, and anyone else who comes near the Holy Place will be put to death.”