Otzar ha-Geonim, ed. B. Lewin, vol. 7: Tractate Yevamot (Jerusalem, 1936), p. 314.

Jewish Mysticism
All the prophets saw [the glory] within the speculum that does not shine, and it seemed to them that they had seen a visible object. This is like an elderly man whose vision is dim and he sees what is low as if it were high and one [thing] as if it were two, and the like, but it is not so. This is what is written, “and spoke parables through the prophets,” u-ve-yad ha-nevi’im ’adammeh (Hosea 12-11), i.e., the vision that they saw was an image (dimyon) and not an actual [entity] (‘iqqar). Moses gazed upon the glory and the splendor of the Shekhinah through the speculum that shines from behind the Shekhinah. He requested [to see] more but it was not granted to him… The view of all is that no creature has been granted permission to see the splendor and the glory that is greater than the glory of the Shekhinah.

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

For a slightly different version see Judah ben Barzillai, Perush Sefer Yesirah, p. 12.