Albert Barnes 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)

Hebrew writers have generally regarded this as a formal injunction to write the precepts on slips of parchment and to fasten them on the wrists and forehead; but other commentators are generally agreed that it is to be understood metaphorically.

The words appear to be put into the mouths of the parents. They were to keep all the facts of the Passover constantly in mind and, in a manner that alluded to a custom prevalent ages before Moses in Egypt, to have them present as though they were inscribed on papyrus or parchment fastened on the wrists or on the face between the eyes.

If, as may be inferred from Deuteronomy 6:7-8, Moses adopted this custom, he would take care to warn the people against the Egyptian superstition of amulets. Modern Israelites generally allege this precept as a justification for the use of phylacteries.