Charles Ellicott Commentary Exodus 13:9

"And it shall be for a sign unto thee upon thy hand, and for a memorial between thine eyes, that the law of Jehovah may be in thy mouth: for with a strong hand hath Jehovah brought thee out of Egypt." — Exodus 13:9 (ASV)

(9) It shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes. —The practice of wearing tephillin, or “phylacteries,” is referred by the Jews themselves to the time of the Exodus and regarded by them as resting on the present passage, together with Deuteronomy 6:8 and Deuteronomy 11:18.

These phylacteries consist of small strips of parchment, on which are written certain passages from the Law—namely, Exodus 13:2–10; Deuteronomy 6:4–9, and Deuteronomy 11:13-21. These strips are then folded tightly, placed in small boxes, and attached by bands to the left wrist and the forehead at the hours of prayer. It is well known that a similar custom prevailed in Egypt (Wilkinson, Ancient Egyptians, vol. iii, p. 364); this has been made an objection to the Mosaic institution of phylacteries, as it has been thought that Moses would not have encouraged an Egyptian superstition.

But the adoption of Egyptian customs, purged from their superstition, is quite in the spirit of the Mosaic institutions and is in no way reprehensible. If the Israelites were inclined to wear amulets, like the Egyptians, it would have been a wise provision to substitute the solemn words of the Law for the magic charms of sorcerers, and in this way to turn a current superstition to good account. By this means, the Law was honored, and the special passages selected would become generally known to those who wore them, and be in their mouth and in their heart (Deuteronomy 11:18).

Dean Plumptre, in his Commentary on the Temptation (Matthew), notes that our Blessed Lord used quotations from the Scriptures that form these very Tephillin against the adversary.