Exodus 17:8-13

And Amalek came and fought with Israel in Rephidim. And Moses said to Joshua, Choose us men, and go out, fight with Amalek; to-morrow I will stand on the top of the hill with the staff of God in my hand. And Joshua did as Moses had said to him, to fight with Amalek; and Moses, Aaron and Hur went up to the top of the hill. And it came to pass when Moses raised his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed. And Moses' hands were heavy; then they took a stone, and put it under him, and he sat on it; and Aaron and Hur supported his hands, one on this side, and one on that side; and his hands were steady until the going down of the sun. And Joshua broke the power of Amalek and his people with the edge of the sword.