Exodus 13:14-15

And it shall be when your son asks you in time to come, saying, ‘What is this?’ then you shall say to him, ‘With a powerful hand the LORD brought us out of Egypt, from the house of slavery. It came about, when Pharaoh was stubborn about letting us go, that the LORD killed every firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore, I sacrifice to the LORD the males, the first offspring of every womb, but every firstborn of my sons I redeem.’