Exodus 17:5-6

17:5 The Lord said to Moses, “Go over before the people; take with you some of the elders of Israel and take in your hand your staff with which you struck the Nile and go. 17:6 I will be standing before you there on the rock in Horeb, and you will strike the rock, and water will come out of it so that the people may drink.” And Moses did so in plain view of the elders of Israel.

Exodus 17:9-12

17:9 So Moses said to Joshua, “Choose some of our men and go out, fight against Amalek. Tomorrow I will stand on top of the hill with the staff of God in my hand.”

17:10 So Joshua fought against Amalek just as Moses had instructed him;and Moses and Aaron and Hur went up to the top of the hill. 17:11 Whenever Moses would raise his hands, then Israel prevailed, but whenever he would rest his hands, then Amalek prevailed. 17:12 When the hands of Moses became heavy, they took a stone and put it under him, and Aaron and Hur held up his hands, one on one side and one on the other, and so his hands were steady until the sun went down.