Numbers 18:15-17

15Every thing that openeth the womb in all flesh, which they bring to the LORD, whether of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem. 16And those that are to be redeemed from a month old shalt thou redeem, according to thy estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs. 17But the firstling of a cow, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem; they are holy: thou shalt sprinkle their blood upon the altar, and shalt burn their fat for an offering made by fire, for a sweet savour to the LORD.