Numbers 18:15-17

¹⁵Everything that openeth the womb, of all flesh which they offer unto Jehovah, both of man and beast shall be thine: nevertheless the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.

¹⁶And those that are to be redeemed of them from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary (the same 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 savor unto Jehovah.