Exodus 13:8-10

8And thou shalt shew thy son in that day, saying, This is done because of that which the Lord did unto me when I came forth out of Egypt. 9And it shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes, that the Lord’s law may be in thy mouth: for with a strong hand hath the Lord brought thee out of Egypt. 10Thou shalt therefore keep this ordinance in his season from year to year.