Exodus 17:10-12

10So Joshua did as Moses said to him, and fought with Amalek. And Moses, Aaron, and Hur went up to the top of the hill.

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.