Exodus 13:15-16

15And when Pharaoh stubbornly refused to let us go, the LORD killed every firstborn in the land of Egypt, both of man and beast. This is why I sacrifice to the LORD the firstborn male of every womb, but I redeem all the firstborn of my sons.’ 16So it shall serve as a sign on your hand and a symbol on your forehead, for with a mighty hand the LORD brought us out of Egypt.”