Exodus 17:8-12

8And Amalek came and fought with Israel in Rephidim.

9And 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 1God in my hand.

10And 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.

11And it came to pass when Moses raised his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed.

12And 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.