Exodus 13:14-16 (KJV)
14 And it shall be when thy son asketh thee in time to come, saying, What is this? that thou shalt say unto him, By strength of hand the Lord brought us out from Egypt, from the house of bondage:
15 And it came to pass, when Pharaoh would hardly let us go, that the Lord slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the Lord all that openeth the matrix, being males; but all the firstborn of my children I redeem.
16 And it shall be for a token upon thine hand, and for frontlets between thine eyes: for by strength of hand the Lord brought us forth out of Egypt.