Exodus 17:11-12

11As long as Moses held up the staff in his hand, the Israelites had the advantage. But whenever he dropped his hand, the Amalekites gained the advantage.

12Moses’ arms soon became so tired he could no longer hold them up. So Aaron and Hur found a stone for him to sit on. Then they stood on each side of Moses, holding up his hands. So his hands held steady until sunset.