Numbers 18:3-5

And they shall thus attend to your obligation and the obligation of all the tent, but they shall not come near to the furnishings of the sanctuary and the altar, or both they and you will die. They shall be joined with you and attend to the obligations of the tent of meeting, for all the service of the tent; but an outsider may not come near you. So you shall attend to the obligations of the sanctuary and the obligations of the altar, so that there will no longer be wrath on the sons of Israel.