Exodus 17:9-11

And Moses said unto Joshua, Choose us out men, and go out, fight with Amalek: to morrow I will stand on the top of the hill with the rod of God in mine hand. So Joshua did as Moses had said to him, and fought with Amalek: and Moses, Aaron, and Hur went up to the top of the hill. And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed.