Exodus 13:12

13:12 then you must give over to the Lord the first offspring of every womb. Every firstling of a beast that you have – the males will be the Lord’s.

Exodus 13:15

13:15 When Pharaoh stubbornly refused to release us, the Lord killed all the firstborn in the land of Egypt, from the firstborn of people to the firstborn of animals. That is why I am sacrificing to the Lord the first male offspring of every womb, but all my firstborn sons I redeem.’