Numbers 18:2-4

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

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

4And they shall be joined to thee, and keep the charge of the tabernacle of the congregation, for all the service of the tabernacle: and a stranger shall not come near to you.