Exodus 17:13

9 So Moses said to Joshua, “Choose some of our men and go out to fight the Amalekites. Tomorrow I will stand on the hilltop with the staff of God in my hand.”

10 Joshua did as Moses had instructed him and fought against the Amalekites, while Moses, Aaron, and Hur went up to the top of the hill.

11 As long as Moses held up his hands, Israel prevailed; but when he lowered them, Amalek prevailed.

12 When Moses’ hands grew heavy, they took a stone and put it under him, and he sat on it. Then Aaron and Hur held his hands up, one on each side, so that his hands remained steady until the sun went down.

13 So Joshua overwhelmed Amalek and his army with the sword.

14 Then the LORD said to Moses, “Write this on a scroll as a reminder and recite it to Joshua, because I will utterly blot out the memory of Amalek from under heaven.”

15 And Moses built an altar and named it The LORD Is My Banner.

16 “Indeed,” he said, “a hand was lifted up toward the throne of the LORD. The LORD will war against Amalek from generation to generation.”

Read all of Exodus 17