Numbers 18:2-4

2And thy brethren also, the tribe of Levi, the tribe of thy father, bring near with thee, that they may unite with thee, and minister unto thee; but thou and thy sons with thee [shall serve] before the tent of the testimony.

3And they shall keep thy charge, and the charge of the whole tent: only they shall not come near to the vessels of the sanctuary and to the altar, that they may not die, and you as well as they.

4And they shall unite with thee, and keep the charge of the tent of meeting, for all the service of the tent; and no stranger shall come near to you.