And when forty years were expired, there appeared to him in the wilderness of mount Sina an angel of the Lord in a flame of fire in a bush. In a flame of fire in a bush - See this and the following verses largely explained in the notes on Exodus 3:1-8 (note).
Related Commentaries of Acts 7:30
Acts 7:30
30 And when forty years were expired, there appeared to him in the wilderness of mount Sina an angel of the Lord in a flame of fire in a bush.