Numbers 18:2-3

²Let your brothers, the family of Levi, come near with you, so that they may be joined with you and be your servants: but you and your sons with you are to go in before the ark of witness.

³They are to do your orders and be responsible for the work of the Tent; but they may not come near the vessels of the holy place or the altar, so that death may not overtake them or you.