Exodus 17:11-13

11As long as Moses held up his hands, the Israelites were winning, but whenever he lowered his hands, the Amalekites were winning. 12When 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.

13So Joshua overcame the Amalekite army with the sword.