Exodus 17:11

Whenever Moses held up his hand , Israel prevailed , and whenever he lowered his hand , Amalek prevailed .

As long as Moses held up his hands, the Israelites were winning, but whenever he lowered his hands, the Amalekites were winning.

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