Exodus 13:14-16

14“In the future, when your son asks you, ‘What does this mean?’ say to him, ‘By the strength of his hand the Lord brought us out of Egypt, out of the place of slavery.

15When Pharaoh stubbornly refused to let us go, the Lord killed every firstborn male in the land of Egypt, both the firstborn of humans and the firstborn of livestock. That is why I sacrifice to the Lord all the firstborn of the womb that are males, but I redeem all the firstborn of my sons.’

16So let it be a sign on your hand and a symbol on your forehead, for the Lord brought us out of Egypt by the strength of his hand.”