Mekhilta de-Rabbi Ishmael, Bahodesh, 2, p. 211

Jewish Mysticism
They [the Israelites] said- Our wish is to see our king; one who hears is not comparable to one who sees. God said to him [Moses]- Give them what they have requested, “For on the third day the Lord will descend in the sight of all the people upon Mount Sinai” (Exod. 19-11).

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