Exodus 17:11-12

11And so it was, when Moses held up his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed.

12But Moses’ hands became heavy; so 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 the other on the other side; and his hands were steady until the going down of the sun.