Numbers 18:2-3

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.