Numbers 18:2-4

2And with you bring your brothers also, the tribe of Levi, the tribe of your father, that they may join you and minister to you while you and your sons with you are before the tent of the testimony.

3They shall keep guard over you and over the whole tent, but shall not come near to the vessels of the sanctuary or to the altar lest they, and you, die.

4They shall join you and keep guard over the tent of meeting for all the service of the tent, and no outsider shall come near you.