“Now when he [Moses] was forty years old, it came into his heart to visit his brethren, the children of Israel.” – Acts 7:23
“And afterwards, when thou wast grown up, they brought thee unto the daughter of Pharaoh, and thou didst become her son, and Amram thy father taught thee writing, and after thou hadst completed three weeks they brought thee into the royal court. And thou wast three weeks of years at court until the time when thou didst go forth from the royal court and didst see an Egyptian smiting thy friend who was of the children of Israel, and thou didst slay him and hide him in the sand.” – Book of Jubilees 47:9-11
The Scriptural account of the Prophet Moses visiting his people and killing the Egyptian to protect one of them (Exodus 2:11-15) states that it happened only when Moses “was grown,” not giving an exact age. Jubilees tells us that it happened after two sets of “three weeks of years” (1 week = 7 days; 7 x 3 = 21; 21 x 2 = 42), indicating that he was 42 years old. St. Stephen (via St. Luke) tells us in the verse quoted above that Moses was 40. There’s a number of reasons why it’s unlikely that St. Stephen was referencing Jubilees itself here (including the two year discrepancy in ages given), but what can be deduced (as with Acts 7:16 above and Acts 7:30 below, all from St. Stephen’s speech) is that both St. Stephen and Jubilees are drawing on a common Jewish oral tradition. And, for our purposes here, all that matters is that it is an extra-biblical tradition.