Zohar 2:129a

Jewish Mysticism
It is necessary to fall on one’s face [during the prayer of supplication]. What is the reason? For at that moment it is the time of sexual union, and every person must be ashamed before his Master and cover his face with great shame, and contain his soul in that union of souls.

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.