Eleazar of Worms, MS Paris, BN 772, fols. 157b-158a.

Jewish Mysticism
The word of God is like white fire in a black and dark cloud … thus it was revealed in a cloud that surrounds the glory…”Upon Your holy nation,” at that time [at Sinai] Israel were holy, as it is written, “a holy nation” (Exod. 19-6), for they separated from their wives for three days and were like the ministering angels. Therefore His glory was revealed in order to speak to them. The word that goes out from the mouth of God is a fire brighter in its whiteness than any other fire in the world, and the brightness blinds the eyes like one who looks at the sun when it is in its strength. Therefore the glory, the will of His word, is fire, the form of a cloud and darkness surround it. According to the needs of the hour the word goes out, for the Holy One, blessed be He, places the Presence of His throne of glory between the dark waters, and the Presence is in the clouds.

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