...למען נאמן בית...סלע הך..., “…In the merit of Moshe…who struck the rock…” (Piyut – תענה אמונים)
Rav Shimon Schwab, zt”l, asks, “How can it be that when we are trying to arouse the merit of Moshe and Aharon, we mention חטא מי מריבה (the sin of Moshe’s hitting the rock)?”
Rav Schwab explains that the answer may be found in Parshat Chukat (20:8). Moshe and Aharon are specifically instructed to speak to the rock to provide water to the people and their animals (העדה ואת בעירם). Moshe Rabbeinu may have judged the word ואת to mean that Hashem was differentiating between the type of water Bnai Yisrael were to drink and the type of water to be provided for their animals. He may have thought that Hashem wanted to differentiate between Tzaddikim and Reshaim, between Ovedei Hashem and complainers, in order to eliminate the complainers and the Reshaim. How? While the animals would drink regular water, Bnai Yisrael were to drink מי סוטה, the water a suspected adulteress drinks to prove her innocence or guilt.
Tens of thousands of Bnai Yisrael had already died in the desert as a result of rebellions and plagues. These מי סוטה would surely have added to the number. A leader has to be Moser Nefesh (literally, give over his soul) for his people. Moshe Rabbeinu, therefore, purposefully transgressed Hashem’s command by hitting instead of speaking to the rock, forfeiting his opportunity to enter Eretz Yisrael, in order to save Bnai Yisrael from the מי סוטה. Support for this may be found when Bnai Yisrael and their animals drank; the Pasuk states that the animals and the people drank together, ותשת.
Perhaps now we can understand why Moshe told Bnai Yisrael (Devarim 1:37) that Hashem was not allowing him to enter Eretz Yisrael בגללכם, “because of you.” Moshe, as a leader of Klal Yisrael, may have obligated himself to hit the rock in order to save the remnant of Klal Yisrael threatened by מי סוטה. The merit of this kind of leadership manifests itself on הושענה רבה in the Piyut תענה אמונים.