Or HaChaim on Exodus 17:9/אור החיים על שמות י״ז:ט׳

1 ויאמר משה אל יהושע, Moses said to Joshua, etc. As soon as Moses realised that the sin for which the Israelites were being punished was their neglect of Torah he said to himself that the most suitable person to confront Amalek was Joshua. The Torah itself testifies in Exodus 33,11 that Joshua did not move from the tent as he was busy studying Torah. (the word אהל is traditionally equated with the "tent in which one studies Torah). Moses instructed Joshua to choose people of similar stature in order to overcome Amalek. It turned out that this strategy worked.