Exodus 17:11-13

As long as Moses held up his hands, the Israelites were winning, but whenever he lowered his hands, the Amalekites were winning. When Moses’ hands grew tired, they took a stone and put it under him and he sat on it. Aaron and Hur held his hands up – one on one side, one on the other – so that his hands remained steady till sunset. So Joshua overcame the Amalekite army with the sword.