Numbers 18:2-4

2But bring with you also your brothers from the tribe of Levi, the tribe of your father, that they may join you and assist you and your sons before the Tent of the Testimony. 3And they shall attend to your duties and to all the duties of the Tent; but they must not come near to the furnishings of the sanctuary or the altar, or both they and you will die. 4They are to join you and attend to the duties of the Tent of Meeting, doing all the work at the Tent; but no outsider may come near you.