Exodus 17:8-12

Then Amalek came and fought against Israel at Rephidim.

So Moses said to Joshua, “Choose men for us and go out, fight against Amalek. Tomorrow I will station myself on the top of the hill with the staff of God in my hand.”

Joshua did just as Moses told him, and fought against Amalek; and Moses, Aaron, and Hur went up to the top of the hill.

So it came about, when Moses held his hand up, that Israel prevailed; but when he let his hand down, Amalek prevailed.

And Moses’ hands were heavy. So they took a stone and put it under him, and he sat on it; and Aaron and Hur supported his hands, one on one side and one on the other. So his hands were steady until the sun set.