Exodus 17:8-12

Then Amalek came and fought with Israel at Rephidim. So Moses said to Joshua, “Choose for us men, and go out and fight with Amalek. Tomorrow I will stand on the top of the hill with the staff of God in my hand.” So Joshua did as Moses told him, and fought with Amalek, while Moses, Aaron, and Hur went up to the top of the hill. Whenever Moses held up his hand, Israel prevailed, and whenever he lowered his hand, Amalek prevailed. 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.