Exodus 17:8-16

8 Then `Amalek came and fought with Yisra'el in Refidim.
9 Moshe said to Yehoshua, "Choose men for us, and go out, fight with `Amalek. Tomorrow I will stand on the top of the hill with God's rod in my hand.
10 So Yehoshua did as Moshe had told him, and fought with `Amalek; and Moshe, Aharon, and Hur went up to the top of the hill.
11 It happened, when Moshe held up his hand, that Yisra'el prevailed; and when he let down his hand, `Amalek prevailed.
12 But Moshe' hands were heavy; and they took a stone, and put it under him, and he sat on it. Aharon and Hur held up his hands, the one on the one side, and the other on the other side. His hands were steady until sunset.
13 Yehoshua defeated `Amalek and his people with the edge of the sword.
14 The LORD said to Moshe, "Write this for a memorial in a book, and rehearse it in the ears of Yehoshua: that I will utterly blot out the memory of `Amalek from under the sky."
15 Moshe built an altar, and called the name of it the LORD our Banner
16 He said, "The LORD has sworn: 'The LORD will have war with `Amalek from generation to generation.'"