Sh'mot (Exo) 17:11-12

When Moshe raised his hand, Isra’el prevailed; but when he let it down, ‘Amalek prevailed. However, Moshe’s hands grew heavy; so they took a stone and put it under him, and he sat on it. Aharon and Hur held up his hands, the one on the one side and the other on the other; so that his hands stayed steady until sunset.