Numbers 18:15-19

Every firstborn of the womb of all flesh, whether human or animal, which they offer to the Lord, shall be yours; however you must redeem the human firstborn, and the firstborn of unclean animals you shall redeem.

As to their redemption price, from a month old you shall redeem them, by your assessment, five shekels in silver by the shekel of the sanctuary, which is twenty gerahs.

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.”