Exodus 17:10 (KJV)

Then came Amalek, and fought with Israel in Rephidim.

And Moses said unto Joshua, Choose us out men, and go out, fight with Amalek: to morrow I will stand on the top of the hill with the rod of God in mine hand.

So Joshua did as Moses had said to him, and fought with Amalek: and Moses, Aaron, and Hur went up to the top of the hill.

And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed.

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.