Numbers 18:17-21

But the firstborn of an ox, the firstborn of a sheep, or the firstborn of a goat, you shall not redeem; they are holy. You shall sprinkle their blood on the altar and offer up their fat in smoke as an offering by fire, for a soothing aroma to the Lord.

However, their meat shall be yours; it shall be yours like the breast of a wave offering and like the right thigh.

All the offerings of the holy gifts, which the sons of Israel offer to the Lord, I have given to you and your sons and your daughters with you, as a permanent allotment. It is a permanent covenant of salt before the Lord to you and your descendants with you.”

Then the Lord said to Aaron, “You shall have no inheritance in their land nor own any portion among them; I am your portion and your inheritance among the sons of Israel.

“To the sons of Levi, behold, I have given all the tithe in Israel as an inheritance, in return for their service which they perform, the service of the tent of meeting.