What is the Torah referring to when it writes "Ka'asher Nisisem ba'Masah"?
Rashi: It refers to when, in connection with the lack of water - Yisrael declared "ha'Yesh Hashem be'Kirbeinu Im Ayin (Beshalach Sh'mos, 17:7)?"
Refer to Bamidbar 14:22:1:1. (It seems that the sin of the Meraglim was the tenth in the Pasuk there, and there were more afterwards - refer to Bamidbar 14:22:151:2. Perhaps the Targum says ten, like the Pasuk, but it does not exclude others
Why does the Torah add the (otherwise superfluous) phrase "Ka'asher Nisisem ba'Masah"?
Oznayim la'Torah: It is a gentle hint at the reuslt of testing Hashem - Amalek - aas occurred when they tested him then.
What is the gist of the Mitzvah?
Ramban #1: Not to test Hakadosh-Baruch-Hu. 1
Ramban #2: Not to serve Hashem on condition that He fulfils our requests, and to forsake Him, if He doesn't. 2
Ramban: Since that is what Yisrael really meant when they tested Hashem in Masa - in that if Hashem would supply them with water in a miraculous manner, they would follow Him in the desert ... . This might have been acceptable before Moshe's status as a Navi had been proven, but once it was, querying him - and subsequent prophets - became akin to querying Hashem Himself. See Ramban DH 'Lo Senasu" who elaborates.