When Moses’ hands grew heavy, they took a stone and put it under him, and he sat on it. Then Aaron and Hur held his hands up, one on each side, so that his hands remained steady until the sun went down.
Jewish Publication Society of America Version (JPS1917) - Exodus 17:12 >But Moses' hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.