Exodus 13:15-16

and it came to pass, when Pharaoh would hardly let us go, that Jehovah slew all the first-born in the land of Egypt, both the first-born of man, and the first-born of beast: therefore I sacrifice to Jehovah all that openeth the womb, being males; but all the first-born of my sons I redeem. And it shall be for a sign upon thy hand, and for frontlets between thine eyes: for by strength of hand Jehovah brought us forth out of Egypt.