Joseph of Hamadan, Sefer Tashak.

Jewish Mysticism
The covenant (berit) that is seen in the cloud on a rainy day signifies the colors of the attribute of the Saddiq… Ezekiel the prophet, may peace be upon him, alluded to this when he said, “Like the appearance of the bow which shines in the clouds on a day of rain, such was the appearance of the surrounding radiance. The was the appearance of the semblance of the Presence of the Lord. When I beheld it, I flung myself down on my face” (1-28). From here [it is deduced that] it is forbidden to look at the phallus (berit) and the one who sees the phallus should fall on his face.

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