Exodus 17:11-12

11And it came to pass when Moses raised his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed.

12And 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 this side, and one on that side; and his hands were steady until the going down of the sun.