Exodus 17:5-6

5The LORD answered Moses, “Go out in front of the people. Take with you some of the elders of Israel and take in your hand the staff with which you struck the Nile, and go. 6I will stand there before you by the rock at Horeb. Strike the rock, and water will come out of it for the people to drink.” So Moses did this in the sight of the elders of Israel.