Шмот 28:36 Галаха: Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll

וְעָשִׂ֥יתָ צִּ֖יץ זָהָ֣ב טָה֑וֹר וּפִתַּחְתָּ֤ עָלָיו֙ פִּתּוּחֵ֣י חֹתָ֔ם קֹ֖דֶשׁ לַֽיהוָֽה׃

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Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll

As long as one is wearing phylacteries, he should frequently touch them with his fingers, so as not to lose consciousness of them for a single moment. For their degree of sanctity is higher than that of the golden plate which the High Priest wore on his forehead. On the latter, the name of God was engraved once only, while the phylactery for the head contains the tetragrammaton twenty-one times, and the phylactery for the arm, an equal number of times.