Deuteronomy 6:8

And thou shalt bind them for a sign upon thine hand
As a man ties anything to his hand for a token, that he may remember somewhat he is desirous of; though the Jews understand this literally, of binding a scroll of parchment, with this section and others written in it, upon their left hand, as the Targum of Jonathan here interprets the hand:

and they shall be as frontlets between thine eyes;
and which the same Targum interprets of the Tephilim, or phylacteries, which the Jews wear upon their foreheads, and on their arms, and so Jarchi; of which (See Gill on Matthew 23:5).