Numbers 18:7 (KJV)

And 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.

And 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.

Therefore thou and thy sons with thee shall keep your priest’s office for every thing of the altar, and within the vail; and ye shall serve: I have given your priest’s office unto you as a service of gift: and the stranger that cometh nigh shall be put to death.

And the Lord spake unto Aaron, Behold, I also have given thee the charge of mine heave offerings of all the hallowed things of the children of Israel; unto thee have I given them by reason of the anointing, and to thy sons, by an ordinance for ever.

This shall be thine of the most holy things, reserved from the fire: every oblation of theirs, every meat offering of theirs, and every sin offering of theirs, and every trespass offering of theirs, which they shall render unto me, shall be most holy for thee and for thy sons.