Exodus 17:9-11

So Moses said to Joshua, “Choose some men and go and fight the Amalekites. Tomorrow I will stand on the top of the hill, holding the walking stick of God in my hands.” Joshua obeyed Moses and went to fight the Amalekites, while Moses, Aaron, and Hur went to the top of the hill. As long as Moses held his hands up, the Israelites would win the fight, but when Moses put his hands down, the Amalekites would win.