Numbers 18:2-3

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.