Exodus Rabbah 29:3

Jewish Mysticism
R. Levi said- Israel asked two things of the Holy One, blessed be He–that they might see His glory (kevodo) and hear His voice. And they did see His Glory and hear His voice, as it is written, “The Lord our God has just shown us His majestic Presence, and we have heard His voice out of the fire” (Deut. 5-21).

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