Exodus 17
5 Ref 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 Ref 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.