Exodus 17:11-13

11So it came about, when Moses held his hand up, that Israel prevailed; but when he let his hand 1down, Amalek prevailed. 12And Moses’ hands were heavy. So they took a stone and put it under him, and he sat on it; and Aaron and Hur 2supported his hands, one on one side and one on the other. So his hands were steady until the sun set. 13And Joshua 3defeated Amalek and his people with the edge of the sword.