Numbers 18:15-17

¹⁵Every thing that openeth the womb, of all flesh which they offer unto the LORD, both of man and beast, shall be thine; howbeit the first-born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.

¹⁶And their redemption-money — from a month old shalt thou redeem them — shall be, according to thy valuation, five shekels of silver, after the shekel of the sanctuary — the same is twenty gerahs.

¹⁷But the firstling of an ox, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem; they are holy: thou shalt dash their blood against the altar, and shalt make their fat smoke for an offering made by fire, for a sweet savour unto the LORD.