Numbers 18:5-7

“You are to guard the sanctuary and the altar so that wrath may not fall on the Israelites again.  Look, I have selected your fellow Levites from the Israelites as a gift for you, assigned by the LORD to work at the tent of meeting. But you and your sons will carry out your priestly responsibilities  for everything concerning the altar and for what is inside the curtain, and you will do that work. I am giving you the work of the priesthood as a gift,  but an unauthorized person who comes near the sanctuary will be put to death.”