Charles Ellicott Commentary 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." — Exodus 17:12 (ASV)

Moses’ hands were heavy. —Moses writes with a clear remembrance of his feelings at the time. His hands, long stretched to heaven, grew weary, “heavy,” feeble; he could no longer raise them up, much less stretch them out, by his own muscular energy. They sank down and dropped by his sides. So that the battle would not be lost, it was necessary to find some remedy. Apparently, Aaron and Hur thought of an effective remedy, with no suggestion from Moses.

They took a stone. —Partly to give him a certain amount of rest, but perhaps mainly to enable them to better sustain his hands. The fact is one of those “little” ones, which only someone involved in the events would likely have known. (See “Introduction,” § 5)

Aaron and Hur stayed up his hands. —Left to himself, Moses had become exhausted both mentally and bodily, and when his hands dropped, he had ceased to pray. Sustained physically by his two companions, his mind recovered, and he was able to renew his supplications and continue them. The result was the victory.