Exodus 13:15-16

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.’ So it shall serve as a sign on your hand and as phylacteries on your forehead, for with a powerful hand the LORD brought us out of Egypt.”