26 “And they angered Him at the waters of strife: so that Moses was vexed for their sakes”; “because they provoked his spirit, so that he spoke doubtfully with his lips”. What is spoke doubtfully? As if God, who had done so great wonders before, could not cause water to flow from a rock. For he touched the rock with his rod with doubt, and thus distinguished this miracle from the rest, in which he had not doubted. He thus offended, thus deserved to hear that he should die, without entering into the land of promise. For being disturbed by the murmurs of an unbelieving people, he held not fast that confidence which he ought to have held. Nevertheless, God gives unto him, as unto His chosen, a good testimony even after his death, so that we may see that this wavering of faith was punished with this penalty only, that he was not allowed to enter that land, whither he was leading the people....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)