Exodus 17:10-12

So Joshua fought the Amalekites as Moses had ordered, and Moses, Aaron and Hur went to the top of the hill. 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.