Exodus 13:15-16

For when Pharaoh stubbornly refused to let us go, the LORD killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of animals. Therefore I sacrifice to the LORD all the males that first open the womb, but all the firstborn of my sons I redeem.’ It shall be as a mark on your hand or frontlets between your eyes, for by a strong hand the LORD brought us out of Egypt.”