According to Exodus 17, Aaron and Hur helped hold up Moses’s hands during Israel’s battle with Amalek.
The battle took place at Rephidim after the Israelites had left Egypt. Moses stood on a hill with the staff of God in his hand. Whenever he raised his hands, Israel prevailed; when his hands became tired and fell, Amalek gained strength.
Aaron and Hur supported Moses by seating him on a stone and holding up his hands, one on each side, until sunset. Joshua led the Israelites in the fighting below, while Moses, Aaron, and Hur occupied the hill.
This episode is distinct from the later battle commander question: Joshua led the army, but Aaron and Hur physically supported Moses during the decisive part of the contest.