Exodus 17:9-11

9Moses said to Joshua, “Choose some of our men and go out to fight the Amalekites. Tomorrow I will stand on top of the hill with the staff of God in my hands.” 10So Joshua fought the Amalekites as Moses had ordered, and Moses, Aaron and Hur went to the top of the hill. 11As long as Moses held up his hands, the Israelites were winning, but whenever he lowered his hands, the Amalekites were winning.