Exodus 17:11–12

11 Whenever Moses held up his hand, Israel prevailed; and whenever he lowered his hand, Amalek prevailed. 12 But Moses’ hands grew weary; so they took a stone and put it under him, and he sat upon it, and Aaron and Hur held up his hands, one on one side, and the other on the other side; so his hands were steady until the going down of the sun.