Numbers 18:2-4

2And thy brethren also, the tribe of Levi, the tribe of thy father, bring thou near with thee, that they may be joined unto thee, and minister unto thee: but thou and thy sons with thee shall be before the tent of the testimony.

3And they shall keep thy charge, and the charge of all the Tent: only they shall not come nigh unto the vessels of the sanctuary and unto the altar, that they die not, neither they, nor ye.

4And they shall be joined unto thee, and keep the charge of the tent of meeting, for all the service of the Tent: and a stranger shall not come nigh unto you.