Exodus 17:11-12

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.