Numbers 18:5-7

5And ye shall keep the charge of the sanctuary, and the charge of the altar; that there come no wrath any more upon the children of Israel.

6And I, behold, I have taken your brethren, the Levites, from among the children of Israel; to you are they given as a gift for Jehovah to perform the service of the tent of meeting.

7But thou and thy sons with thee shall attend to your priesthood for all that concerneth the altar, and for that which is inside the veil; and ye shall perform the service: I give you your priesthood as a service of gift, and the stranger that cometh near shall be put to death.