Exodus 17:11-12

As long as Moses held his hands up, the Israelites would win the fight, but when Moses put his hands down, the Amalekites would win. Later, when Moses’ arms became tired, the men put a large rock under him, and he sat on it. Then Aaron and Hur held up Moses’ hands—Aaron on one side and Hur on the other. They kept his hands steady until the sun went down.