Exodus 17:11-12

and it hath come to pass, when Moses lifteth up his hand, that Israel hath been mighty, and when he letteth his hands rest, that Amalek hath been mighty. And the hands of Moses [are] heavy, and they take a stone, and set [it] under him, and he sitteth on it: and Aaron and Hur have taken hold on his hands, on this side one, and on that one, and his hands are stedfast till the going in of the sun