Exodus 17:11; Exodus 17:12

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 on it, while 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.