Exodus 13:14-16

And it shall be when your son asks you in time to come, saying, ‘What does this mean?’ you shall say to him, ‘With a strong and powerful hand the LORD brought us out of Egypt, from the house of bondage and slavery. For it happened, when Pharaoh stubbornly refused to let us go, that the LORD struck every firstborn in the land of Egypt, both the firstborn of man and the firstborn of animal. Therefore, I sacrifice to the LORD all the males, the first [to be born] of every womb, but every firstborn of my sons I redeem.’ So it shall serve as a sign and a reminder on your [left] hand (arm) and as frontlets between your eyes, for by a strong and powerful hand the LORD brought us out of Egypt.”