Exodus 17:11-13

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. So Joshua overwhelmed Amalek and his people with the edge of the sword.