Rashi on Numbers 20:11/רש"י על במדבר כ׳:י״א

1פעמים [HE SMOTE THE ROCK] TWICE, because at the first attempt it did not bring forth more than a few drops, for God had not bidden him smite it, but He had said, (v. 8) “and ye shall speak to the rock”. They had, indeed, spoken, but to a different rock (not that which God had intended) and it had not given forth water. They said, “Perhaps it is necessary to smite it as on the former occasion when it says, (Exodus 17:6) ‘and ye shall smite the rock’, and just that rock intended by God happened to be there and they smote it [but without full effect, and so they smote it a second time] (cf. Midrash Tanchuma, Chukat 9).