Exodus 17:9-12

So Moses said to Joshua, “Choose men for us and go out, fight against Amalek [and his people]. Tomorrow I will stand on the top of the hill with the staff of God in my hand.” So Joshua did as Moses said, and fought with Amalek; and Moses, Aaron, and Hur went up to the hilltop. 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.