Exodus 13:2, 12-15

“Dedicate to me every firstborn among the Israelites. The first offspring to be born, of both humans and animals, belongs to me.”

you must present all firstborn sons and firstborn male animals to the LORD, for they belong to him. A firstborn donkey may be bought back from the LORD by presenting a lamb or young goat in its place. But if you do not buy it back, you must break its neck. However, you must buy back every firstborn son. “And in the future, your children will ask you, ‘What does all this mean?’ Then you will tell them, ‘With the power of his mighty hand, the LORD brought us out of Egypt, the place of our slavery. Pharaoh stubbornly refused to let us go, so the LORD killed all the firstborn males throughout the land of Egypt, both people and animals. That is why I now sacrifice all the firstborn males to the LORD—except that the firstborn sons are always bought back.’