Numbers 18:15-17

The firstborn of every living thing, human or animal, presented to the LORD belongs to you. But you must certainly redeem a human firstborn,  and redeem the firstborn of an unclean animal. You will pay the redemption price for a month-old male according to your assessment: five shekels  of silver by the standard sanctuary shekel, which is twenty gerahs.  “However, you must not redeem the firstborn of an ox, a sheep, or a goat; they are holy. You are to splatter their blood on the altar and burn their fat as a food offering for a pleasing aroma to the LORD.