The passage you are likely thinking of is in
Exodus 17:12.
It describes how Aaron and Hur held up Moses' hands during a battle against the Amalekites:
"But Moses’ hands grew weary, so they took a stone and put it under him, and he sat on it, and Aaron and Hur held up his hands, one on one side, and the other on the other side. So his hands were steady until the going down of the sun."