For at that juncture, when Moses was ordered to go down into Egypt and lead out the people of the Israelites who were there, and while he was tending the flocks of his maternal uncle in the land of Arabia, our Christ conversed with him under the appearance of fire from a bush, and said, “Put off thy shoes, and draw near and hear.” And he, when he had put off his shoes and drawn near, heard that he was to go down into Egypt and lead out the people of the Israelites there; and he received mighty power from Christ, who spoke to him in the appearance of fire, and went down and led out the people, having done great and marvelous things; which, if you desire to know, you will learn them accurately from his writings.
Justin Martyr
First Apology
Chapter LXII