Numbers 18:20

Numbers 18 defines the responsibilities and support of the priesthood: Aaron and his sons bear the iniquity of the sanctuary and priesthood, while the Levites assist them but must not come near the sacred vessels or altar on pain of death. The priests' portion is the most holy offerings, the wave offerings, and every devoted thing, given as a perpetual due to Aaron and his family. The Levites receive the tithes of Israel as their inheritance in return for their service, and they in turn must present a tithe of that tithe to the Lord as a heave offering.