Exodus 17:8-16 New King James Version (NKJV)
Victory over the Amalekites
8 Now Amalek came and fought with Israel in Rephidim. 9 And Moses said to Joshua, “Choose us some men and go out, fight with Amalek. Tomorrow I will stand on the top of the hill with the rod of God in my hand.” 10 So Joshua did as Moses said to him, and fought with Amalek. And Moses, Aaron, and Hur went up to the top of the hill. 11 And so it was, when Moses held up his hand, that Israel prevailed; and when he let down his hand, Amalek prevailed. 12 But Moses’ hands becameheavy; 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. 13 So Joshua defeated Amalek and his people with the edge of the sword.
14 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.” 15 And Moses built an altar and called its name, The-Lord-Is-My-Banner; 16 for he said, “Because the Lord has sworn: the Lord will have war with Amalek from generation to generation.”