1)
Where did Hashem open the rock and water flowed?
1.
Radak, Malbim: It was in Refidim.
2)
What is the meaning of "Halchu ba'Tziyus Nahar"?
1.
Rashi: Rivers went [out] from the well [that accompanied Yisrael] in a desert. Malbim - initially drops flowed; afterwards, rivers flowed.
2.
Radak (from Bamidbar Rabah 19:26): They went in boats (in the water that came from the well) from one Degel 1 to another. "Ba'Tziyus" is like "v'Tzi Adir" (Yeshayah 33:21).
1
We corrected the text of Radak based on Bamidbar Rabah. (PF)
