Numbers 18:15-18

15Every firstborn male, human or animal, that is brought to the Lord is yours. But you must buy back every firstborn son and the firstborn male of any unclean animal.

16When they are one month old, you must buy them back at the fixed price of two ounces of silver, using the standard weight of the holy place.

17“But you must never buy back a firstborn ox, sheep, or goat. They are holy. Throw the blood from these animals against the altar, and burn the fat as an offering by fire, a soothing aroma to the Lord.

18But the meat is yours, like the breast and the right thigh that are presented.