Numbers 18:2-3

²And take to thyself thy brethren the tribe of Levi, the family of thy father, and let them be joined to thee, and let them minister to thee; and thou and thy sons with thee shall minister before the tabernacle of witness.

³And they shall keep thy charges, and the charges of the tabernacle; only they shall not approach the holy vessels and the altar, so both they and you shall not die.