Numbers 18:5-9

So you shall perform the duties of the sanctuary and the duties of the altar, so that there will no longer be wrath on the sons of Israel.

Behold, I Myself have taken your fellow Levites from among the sons of Israel; they are a gift to you, dedicated to the Lord, to perform the service for the tent of meeting.

But you and your sons with you shall attend to your priesthood for everything that concerns the altar and inside the veil, and you are to perform service. I am giving you the priesthood as a service that is a gift, and the unauthorized person who comes near shall be put to death.” The Priests’ Portion

Then the Lord spoke to Aaron, “Now behold, I Myself have put you in charge of My offerings, all the holy gifts of the sons of Israel I have given to you as a portion and to your sons as a permanent allotment.

This shall be yours from the most holy gifts reserved from the fire; every offering of theirs, namely every grain offering, every sin offering, and every guilt offering, with which they shall make restitution to Me, shall be most holy for you and for your sons.