At Rephidim in the wilderness, the Amalekites attacked the rear of Israel. Moses commanded Joshua: 'Choose men for us and go out, fight with Amalek. Tomorrow I will stand on top of the hill with the rod of God in my hand.' Joshua did so, and Moses, Aaron, and Hur went up the hill.
As long as Moses held up his hand, Israel prevailed; whenever he let it down, Amalek prevailed. When his hands grew weary, they took a stone and put it under him, and he sat upon it. Aaron and Hur held up his hands, one on one side and one on the other; and his hands were steady until the going down of the sun. Joshua mowed down Amalek and his people. Moses built an altar and called its name 'The LORD is my banner' (Yhwh-nissi). The episode is the foundational image of intercessory prayer in Christian tradition.