Numbers 18:18

Everything that opens the womb, of all flesh which they offer to the LORD, both of man and animal, shall be yours. Nevertheless, you shall surely redeem the firstborn of man, and you shall redeem the firstborn of unclean animals.

You shall redeem those who are to be redeemed of them from a month old, according to your estimation, for five shekels of money, according to the shekel of the sanctuary, which weighs twenty gerahs.

“But you shall not redeem the firstborn of a cow, or the firstborn of a sheep, or the firstborn of a goat. They are holy. You shall sprinkle their blood on the altar, and shall burn their fat for an offering made by fire, for a pleasant aroma to the LORD.

All the wave offerings of the holy things which the children of Israel offer to the LORD, I have given you and your sons and your daughters with you, as a portion forever. It is a covenant of salt forever before the LORD to you and to your offspring with you.”

The LORD said to Aaron, “You shall have no inheritance in their land, neither shall you have any portion among them. I am your portion and your inheritance among the children of Israel.

“To the children of Levi, behold, I have given all the tithe in Israel for an inheritance, in return for their service which they serve, even the service of the Tent of Meeting.