Exodus 17:11-14

11Now when Moses held up his hand, Israel prevailed, and when he lowered his hand [due to fatigue], Amalek prevailed.

12But Moses’ hands were heavy and he grew tired. So they took a stone and put it under him, and he sat on it. Then Aaron and Hur held up his hands, one on one side and one on the other side; so it was that his hands were steady until the sun set.

13So Joshua overwhelmed and defeated Amalek and his people with the edge of the sword.

14Then the Lord said to Moses, “Write this in the book as a memorial and recite it to Joshua, that I will utterly wipe out the memory of Amalek [and his people] from under heaven.”