Exodus 17:8-13

Then came Amalek [descendants of Esau] and fought with Israel at Rephidim. And Moses said to Joshua, Choose us out men and go out, fight with Amalek. Tomorrow I will stand on the top of the hill with the rod of God in my hand. So Joshua did as Moses said and fought with Amalek; and Moses, Aaron, and Hur went up to the hilltop. When Moses held up his hand, Israel prevailed; and when he lowered his hand, Amalek prevailed. But Moses' hands were heavy and grew weary. So [the other men] took a stone and put it under him and he sat on it. Then Aaron and Hur held up his hands, one on one side and one on the other side; so his hands were steady until the going down of the sun. And Joshua mowed down and disabled Amalek and his people with the sword.