Numbers 18:15-20

15Every firstborn of the womb of all flesh, whether it is man or animal, which they bring to the Lord, shall be yours; nevertheless, the firstborn of man you shall most certainly redeem, and the firstborn of unclean animals you shall redeem.

16And their redemption price, from a month old you shall redeem, according to your valuation, for the [fixed] price of five shekels in silver, in accordance with the shekel of the sanctuary, which is twenty gerahs.

17But the firstborn of a cow or of a sheep or of a goat you shall not redeem; they are holy [and belong to the Lord]. You shall sprinkle their blood on the altar and shall offer up their fat in smoke as an offering by fire, for a sweet and soothing aroma to the Lord.

18Their meat shall be yours, like the breast of a wave offering and like the right thigh.

19All the offerings of the holy things, which the Israelites offer to the Lord I have given to you and to your sons and your daughters with you as a continual allotment. It is an everlasting covenant of salt [that cannot be dissolved or violated] before the Lord to you and to your descendants with you.”

20Then the Lord said to Aaron, “You shall have no inheritance in the land [of the Israelites], nor have any portion [of land] among them. I am your portion and your inheritance among the children of Israel.