Exodus 17:9

5 And the LORD said to Moses, “Walk on ahead of the people and take some of the elders of Israel with you. Take along in your hand the staff with which you struck the Nile, and go.

6 Behold, I will stand there before you by the rock at Horeb. And when you strike the rock, water will come out of it for the people to drink.” So Moses did this in the sight of the elders of Israel.

7 He named the place Massah and Meribah because the Israelites quarreled, and because they tested the LORD, saying, “Is the LORD among us or not?”

8 After this, the Amalekites came and attacked the Israelites at Rephidim.

9 So Moses said to Joshua, “Choose some of our men and go out to fight the Amalekites. Tomorrow I will stand on the hilltop with the staff of God in my hand.”

10 Joshua did as Moses had instructed him and fought against the Amalekites, while Moses, Aaron, and Hur went up to the top of the hill.

11 As long as Moses held up his hands, Israel prevailed; but when he lowered them, Amalek prevailed.

12 When Moses’ hands grew heavy, they took a stone and put it under him, and he sat on it. Then Aaron and Hur held his hands up, one on each side, so that his hands remained steady until the sun went down.

13 So Joshua overwhelmed Amalek and his army with the sword.

Read all of Exodus 17