Numbers 18:2-3

²and also thy brethren, the tribe of Levi, the tribe of thy father, bring near with thee, and they are joined unto thee, and serve thee, even thou and thy sons with thee, before the tent of the testimony.

³'And they have kept thy charge, and the charge of all the tent; only, unto the vessels of the sanctuary and unto the altar they do not come near, and they die not, either they or you;