Numbers 18:5-7

“Your job is to take care of the Sanctuary and the Altar so that there will be no more outbreaks of anger on the People of Israel. I personally have picked your brothers, the Levites, from Israel as a whole. I’m giving them to you as a gift, a gift of GOD, to help with the work of the Tent of Meeting. But only you and your sons may serve as priests, working around the Altar and inside the curtain. The work of the priesthood is my exclusive gift to you; it cannot be delegated—anyone else who invades the Sanctuary will be executed.”