זוהר שמות — דף צב ע”א

The Zohar begins by explaining "Remember the Shabbat day to sanctify it" as the secret of the holy covenant. Just as the brit mila encompasses all bodily organs, Shabbat encompasses all of the Torah and all its secrets depend on it. The discussion continues on the secret of the two letters of the holy Name that illuminate each other, corresponding to the thirteen attributes of mercy. Rabbi Shimon reveals that when the letters unite and join each other, they shine and grow, and from their union the supernal light is revealed.