Shemot 17:11-12

And it came to pass, as long as Moshe held up his yad, that Yisroel prevailed; and when he let down his yad, Amalek prevailed. When the hands of Moshe grew heavy, they took an even (stone), and put it under him, and he sat thereon; and Aharon and Chur supported his hands, the one on the one side, and the other on the other side; and his hands held emunah (steady) until bo hashemesh (sunset).