Exodus 17:7-11

And he called the name of the place Massah, and Meribah, because of the chiding of the children of Israel, and because they tempted the LORD, saying, Is the LORD among us, or not?

Then came Amalek, and fought with Israel in Rephidim.

And Moses said unto Joshua, Choose us out men, and go out, fight with Amalek: to morrow I will stand on the top of the hill with the rod of God in mine hand.

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

And it came to pass, when Moses held up his hand, that Israel prevailed: and when he let down his hand, Amalek prevailed.