Exodus 17:13

So Joshua did as Moses had told him, and fought with Amalek; and Moses, Aaron, and Hur went up to the top of the hill.

When Moses held up his hand, Israel prevailed. When he let down his hand, Amalek prevailed.

But Moses’ hands were heavy; so they took a stone, and put it under him, and he sat on it. Aaron 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.

The LORD said to Moses, “Write this for a memorial in a book, and rehearse it in the ears of Joshua: that I will utterly blot out the memory of Amalek from under the sky.”

Moses built an altar, and called its name “The LORD our Banner”.

He said, “The LORD has sworn: ‘The LORD will have war with Amalek from generation to generation.’”