Exodus 17:11-12

As long as Moses held up his hand, the Israelites were winning. But every time he lowered his hands, the Amalekites began to win. When Moses’ arms got tired, Aaron and Hur got a stone and put it under him. Then he sat on it. Aaron and Hur held up his hands. Aaron was on one side, and Hur was on the other. Moses’ hands remained steady until sunset.