Numbers 18:5-7

5And ye shall keep the charge of the sanctuary, and the charge of the altar: that there be 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 they are given as a gift for the LORD, to do the service of the tabernacle of the congregation. 7Therefore thou and thy sons with thee shall keep your priest’s office for every thing of the altar, and within the veil; and ye shall serve: I have given your priest’s office to you as a service of gift: and the stranger that cometh near shall be put to death.