Exodus 17:10-12

Joshua did as Moses told him, and fought against Amalek; and Moses, Aaron, and Hur went up to the top of the hill. So it came about when Moses held his hand up, that Israel prevailed, and when he let his hand down, Amalek prevailed. But Moses’ hands were heavy. Then they took a stone and put it under him, and he sat on it; and Aaron and Hur supported his hands, one on one side and one on the other. Thus his hands were steady until the sun set.