Exodus 13:9 - Albert Barnes' Notes on the Bible

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, referring 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.