Exodus 17:11-12

And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed. But Moses' hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.