זוהר שמות — דף רל ע”א

When Jacob went to Haran, he took twelve holy stones that all became one stone. These stones of the place are the secret of the Tabernacle. The breastplate and the ephod correspond to the Urim and Tummim, and this is the secret of the tefillin. Moses saw the back of the Divine Presence when it was said 'you will see My back', and saw the knot of the tefillin. The face is the supreme secret that no person can grasp or comprehend.