Numbers 18:15-17

Everything that opens the womb of all flesh, whether man or beast, which they offer to the LORD, shall be yours. Nevertheless, the firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem. And their redemption price (at a month old you shall redeem them) you shall fix at five shekels in silver, according to the shekel of the sanctuary, which is twenty gerahs. But the firstborn of a cow, or 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 shall burn their fat as a food offering, with a pleasing aroma to the LORD.