Exodus 13:8-10

8You shall explain this to your son on that day, saying, ‘It is because of what the Lord did for me when I came out of Egypt.’

9It shall serve as a sign to you on your hand (arm), and as a reminder on your forehead, so that the instruction (law) of the Lord may be in your mouth; for with a strong and powerful hand the Lord brought you out of Egypt.

10Therefore, you shall keep this ordinance at this time from year to year.