Exodus 17:8, 9, 10, 11, 12, 13

Then Amalek came and waged war against Israel in Rephidim.

So Moses said to Joshua, “Choose some men for us, and tomorrow go out and engage Amalek in battle. I will be standing on top of the hill with the staff of God in my hand.”

Joshua did as Moses told him: he engaged Amalek in battle while Moses, Aaron, and Hur climbed to the top of the hill.

As long as Moses kept his hands raised up, Israel had the better of the fight, but when he let his hands rest, Amalek had the better of the fight.

Moses’ hands, however, grew tired; so they took a rock and put it under him and he sat on it. Meanwhile Aaron and Hur supported his hands, one on one side and one on the other, so that his hands remained steady until sunset.

And Joshua defeated Amalek and his people with the sword.