Numbers 18:6-8

Behold, I Myself have taken your brethren the Levites from among the children of Israel; they are a gift to you, given by the LORD, to do the work of the tabernacle of meeting. Therefore you and your sons with you shall attend to your priesthood for everything at the altar and behind the veil; and you shall serve. I give your priesthood to you as a gift for service, but the outsider who comes near shall be put to death.” And the LORD spoke to Aaron: “Here, I Myself have also given you charge of My heave offerings, all the holy gifts of the children of Israel; I have given them as a portion to you and your sons, as an ordinance forever.