Exodus 17:12-15

But Moses’ hands became 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 the other on the other side; and his hands were steady until the going down of the sun. So Joshua defeated Amalek and his people with the edge of the sword. Then the LORD said to Moses, “Write this for a memorial in the book and recount it in the hearing of Joshua, that I will utterly blot out the remembrance of Amalek from under heaven.” And Moses built an altar and called its name, The-LORD-Is-My-Banner