Rashi on Deuteronomy 2:33/רש"י על דברים ב׳:ל״ג

1ואת בנו [AND WE SMOTE HIM] AND HIS SONS — It is written בנו, "his son" (although read as בניו "his sons"), for he had a son who was as mighty as himself (Midrash Tanchuma, Chukat 25).