Rashi on Exodus 17:5/רש"י על שמות י״ז:ה׳

1עבר לפני העם PASS BEFORE THE PEOPLE and see whether they will stone you! (Midrash Tanchuma, Beshalach 23) Why have you uttered a slander against My children?

2וקח אתך מזקני ישראל AND TAKE WITH THEE OF THE ELDERS OF ISRAEL as witnesses — that they may see that it is by your agency that the water will come forth from the rock and that people may not say that there had been springs there from time immemorial (Mekhilta).

3ומטך אשר הכית בו את היאר AND THY STAFF, WHEREWITH THOU SMOTEST THE RIVER — What is the force of the words “wherewith thou smotest the river” — they are apparently superfluous? But these are added because the Israelites had said of the staff that it was intended only for punishment: by it Pharaoh and the Egyptians had been stricken with many plagues in Egypt and at the Red Sea. On this account it is stated here: take the staff wherewith thou smotest the river — they shall see now that it is also intended for good (Mekhilta).