Numbers 18:15-17

1512Every 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.

16As to 3their redemption price, from a month old you shall redeem them, by your assessment, five 4shekels in silver by the 5shekel of the sanctuary, which is twenty gerahs.

17But 6the firstborn of an ox, the firstborn of a sheep, or the firstborn of a goat, you shall not redeem; they are holy. 7You 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.