From the Rabbula Gospels (Florence, Bib. Laurenziana, cod. Plutt. I, 56. fol. 13v)
Kessler, Introduction to "The Christian Realm: Narrative Representations" (Weitzmann, p. 454), suggests that the use of the chariot is based on the Old Testament reading for Ascension Day in the Syriac liturgy, Ezekiel 1:10.
More of the Ascension