The Amalekites fought Israel at Rephidim.
Moses said to Joshua, "Choose some of our men. Then fight the Amalekites. Tomorrow I will stand on top of the hill. I will hold in my hand the staff God told me to take along."
Joshua did as Moses told him and fought the Amalekites, while Moses, Aaron, and Hur went to the top of the hill.
As long as Moses held up his hands, Israel would win, but as soon as he put his hands down, the Amalekites would start to win.
Eventually, Moses' hands felt heavy. So Aaron and Hur took a rock, put it under him, and he sat on it. Aaron held up one hand, and Hur held up the other. His hands remained steady until sunset.
So Joshua defeated the Amalekite army in battle.