As Ramban points out, it is unclear from the psukim what exactly Moshe and Aharon did wrong in the incident of “Mei Meriva”. Rashi famously brings down from the Midrash that Moshe was meant to speak, not strike the rock. If so, asks Ramban, why was Moshe commanded to take his staff?
The answer can be gleaned from Rashbam and Chizkuni. The staff that Moshe was commanded to bring was not Moshe’s, but rather the staff of Aharon from Parshat Korach. Aharon’s staff blossomed, and would serve as a guide of the futility of rebelling against Hashem and His agents. Bnei Yisrael’s quarreling was quelled when they saw the staff. Hence Moshe’s accusation of שמעו נא המורים was misplaced.
Ramban brings from Rambam that the sin was Moshe getting angry. Ramban asks that the Torah explicitly points out that the sin was יען לא האמנתם בי להקדישני somehow a lack of expression of Emuna on Moshe’s level.
Perhaps the answer is that anger itself connected to a blemish in the area of faith, as chazal compare one who gets angry to Avoda Zara (on some level).
Ramban likes R’ Chananel who writes that Moshe should have attributed the water flow directly to Hashem, as opposed to Moshe and Aharon המן הסלע הזה יוציא ה’ לכם מים as opposed to המן הסלע הזה נוציא לכם מים.
Shabbat Shalom