Why does it say "Al Tikshu Levavchem ki'Mrivah"?
Radak: Do not do like your fathers did.
Malbim: Kishuy Lev is when one sees Hashem's wonders, and hardens his heart against Him - "va'Ani Akasheh Es Lev Pharaoh" (Shemos 7:3). It can apply only to Yisrael, but not to Goyim, for Hashem did not reveal His arm to them with wonders.
What is "k'Yom Masah ba'Midbar"?
Radak: The place where your fathers tested Hashem was called "Masah u'Mrivah Al Riv Bnei Yisrael v'Al Nasosam Es Hashem Leimor ha'Yesh Hashem b'Kirbenu Im Ayin" (Shemos 17:7).
Malbim: [It is when they tested Hashem after the Meraglim returned -] "Ki Kol ha'Anashim ha'Ro'im Es Kevodi v'Es Ososai... va'Ynasu Osi Zeh Eser Pe'amim v'Lo Sham'u b'Koli" (Bamidbar 14:22).
