Exodus 13:15-16

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.”