Deuteronomy 6:8

Thou shalt bind them for a sign upon thy hand As at that time there were few written copies of the whole law, and the people had it read to them only at the feast of tabernacles, God seems to have appointed, at least for the present, that some select sentences of the law, that were most weighty and comprehensive, should literally be written upon their gates and walls, or on slips of parchment, to be worn about their wrists, or bound upon their foreheads. The spirit of the command, however, and the chief thing intended, undoubtedly was, that they should give all diligence and use all means to keep God's laws always in remembrance, as men frequently bind something upon their hands, or put something before their eyes, to prevent forgetfulness of a thing which they much desire to remember.