Exodus 13:14-15

So it shall be, when your son asks you in time to come, saying, ‘What is this?’ that you shall say to him, ‘By strength of hand the LORD brought us out of Egypt, out of the house of bondage. And it came to pass, when Pharaoh was stubborn about letting us go, that the LORD killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore I sacrifice to the LORD all males that open the womb, but all the firstborn of my sons I redeem.’