Exodus 17:8

5 And the Lord said to Moses, Go on before the people, and take some of the chiefs of Israel with you, and take in your hand the rod which was stretched out over the Nile, and go.

6 See, I will take my place before you on the rock in Horeb; and when you give the rock a blow, water will come out of it, and the people will have drink. And Moses did so before the eyes of the chiefs of Israel.

7 And he gave that place the name Massah and Meribah, because the children of Israel were angry, and because they put the Lord to the test, saying, Is the Lord with us or not?

8 Then Amalek came and made war on Israel in Rephidim.

9 And Moses said to Joshua, Get together a band of men for us and go out, make war on Amalek: tomorrow I will take my place on the top of the hill with the rod of God in my hand.

10 So Joshua did as Moses said to him, and went to war with Amalek: and Moses, Aaron, and Hur went up to the top of the hill.

11 Now while Moses' hand was lifted up, Israel was the stronger: but when he let his hand go down, Amalek became the stronger.