Exodus 13:15-16

15It came about, when Pharaoh was stubborn about letting us go, that the 1Lord 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 2offspring of every womb, but every firstborn of my sons I redeem.’ 16So 3it shall 4serve as a sign on your hand and as 5phylacteries 6on your forehead, for with a 7powerful hand the Lord brought us out of Egypt.”