While the people of Israel were still at Rephidim, the warriors of Amalek attacked them. Moses commanded Joshua, “Choose some men to go out and fight the army of Amalek for us. Tomorrow, I will stand at the top of the hill, holding the staff of God in my hand.”
So Joshua did what Moses had commanded and fought the army of Amalek. Meanwhile, Moses, Aaron, and Hur climbed to the top of a nearby hill. As long as Moses held up the staff in his hand, the Israelites had the advantage. But whenever he dropped his hand, the Amalekites gained the advantage. Moses’ arms soon became so tired he could no longer hold them up. So Aaron and Hur found a stone for him to sit on. Then they stood on each side of Moses, holding up his hands. So his hands held steady until sunset. As a result, Joshua overwhelmed the army of Amalek in battle.
After the victory, the LORD instructed Moses, “Write this down on a scroll as a permanent reminder, and read it aloud to Joshua: I will erase the memory of Amalek from under heaven.” Moses built an altar there and named it Yahweh-Nissi (which means “the LORD is my banner”).