And when he was full forty years old, it came into his heart to visit his brethren the children of Israel.
Note 7 at Ac 7:23: This is the only place in Scripture where we are told the age of Moses when he killed the Egyptian (Ex 2:12). Together with Ac 7:30, the only place in Scripture to record the length of time Moses spent in the wilderness, this provides invaluable information about the life of Moses.
Note 8 at Ac 7:23: The words that Stephen chose to use here show that Moses' identification with his Hebrew brethren was not accidental but a deliberate act from his heart. He somehow knew that he was an Israelite. This same deliberate choice on Moses' part is emphasized by the account of Heb 11:24-25.