Shemoth (Exodus) 17:11-12

And it came to be, when Mosheh held up his hand, that Yisra’ĕl prevailed. And when he let down his hand, Amalĕq prevailed. But Mosheh’s hands were heavy, so they took a stone and put it under him, and he sat on it. And Aharon and Ḥur 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.