From the fourteenth-century kabbalist, Joseph ben Shalom Ashkenazi, Sefer Yesirah (Jerusalem, 1962), 27a.

Jewish Mysticism
From the fourteenth-century kabbalist, Joseph ben Shalom Ashkenazi, commenting on the words of Sefer Yesirah 1-8, “Ten ineffable sefirot, their appearance is like lightning,” Commentary on Sefer Yesirah, attributed to the RABaD, in Sefer Yesirah (Jerusalem, 1962), 27a.

There are those who explain [the words] ‘their appearance is like lightning’ [in the following way]- when a prophet or a mystic comes to gaze upon these holy lights [i.e., the sefirot] he knows that at times they shine in relation to him and they appear as if they are lightning, and then they immediately are hidden, and they shine again and are hidden.

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