Tradition attributed to Hai Gaon by Nathan ben Yehiel of Rome in his Talmudic lexicon, Sefer he-‘Arukh Aruch Completum, 1-14

This is explained in Hekhalot Rabbati and Hekhalot Zutarti. They [the mystics] would do certain actions, pray certain prayers in purity, make [theurgic] use of the crown, gaze upon the palaces, see the divisions of angels according to their position and palace after palace… They did not ascend on high but rather in the chamber of their heart they saw and contemplated (ro’in we-sofin be-hadre libban) as a person who sees and contemplates something clearly with his eyes, and they heard and spoke with a seeing eye (‘ein ha-sokheh) by means of the Holy Spirit. This is the explanation of R. Hai Gaon.

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