Exodus 17:9-13

So Moses said to Joshua, “Choose men for us and go out, fight against Amalek. Tomorrow I will station myself on the top of the hill with the staff of God in my hand.”

Joshua did just as Moses told him, and fought against Amalek; and Moses, Aaron, and Hur went up to the top of the hill.

So it came about, when Moses held his hand up, that Israel prevailed; but when he let his hand down, Amalek prevailed.

And Moses’ hands were heavy. So they took a stone and put it under him, and he sat on it; and Aaron and Hur supported his hands, one on one side and one on the other. So his hands were steady until the sun set.

And Joshua defeated Amalek and his people with the edge of the sword.