Numbers 18:6-9

⁶Look, I myself have chosen your brothers the Levites from the Israelites as my gift to you, dedicated to the Lord to do the work that relates the Tent of Meeting.

⁷But only you and your sons are responsible for your priesthood, doing all that concerns the altar and is behind the veil. Only you are to perform that service. I am giving you the gift of your priesthood, but anyone else who approaches the sanctuary must be executed.”

⁸The Lord told Aaron, “Listen, I have put you in charge of officiating at my offerings. All the holy contributions of the Israelites that they bring are reserved for you, and this is a permanent rule.

⁹Part of the most holy offerings taken from the burnt offerings are yours. Part of all the offerings they give me as most holy offerings, whether it's grain offerings or sin offerings or guilt offerings, that part belongs to you and your sons.