זוהר שמות — דף מג ע”ב

The four sections of the tefillin contain the secret of the ten sefirot. The first section, Sanctify to me every firstborn, alludes to Hokhmah, Binah, Hesed and Tiferet. The second section is Binah, the secret of the Jubilee. The third section, Shema Yisrael, is the way of unification in all four directions. The three names of the Shema, God our God God, are three colors unified like a single voice with three tones.