Exodus 17:11-12

Now when Moses held up his hand, Israel prevailed, and when he lowered his hand [due to fatigue], Amalek prevailed. But Moses’ hands were heavy and he grew tired. So they took a stone and put it under him, and he sat on it. Then Aaron and Hur held up his hands, one on one side and one on the other side; so it was that his hands were steady until the sun set.