Numbers 18:5-7

5And ye shall keep the charge of the sanctuary, and the charge of the altar; that there be wrath no more upon the children of Israel. 6And I, behold, I have taken your brethren the Levites from among the children of Israel: to you they are a gift, given unto Jehovah, to do the service of the tent of meeting. 7And thou and thy sons with thee shall keep your priesthood for everything of the altar, and for that within the veil; and ye shall serve: I give you the priesthood as a service of gift: and the stranger that cometh nigh shall be put to death.