Exodus 17:11-13

Young's Literal Translation

¹¹ 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.

¹² 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;

¹³ and Joshua weakeneth Amalek and his people by the mouth of the sword.