Shemot 17:6-7

Hineni, I will stand before thee there upon the tzur in Chorev; thou shalt strike the tzur, and there shall come mayim out of it, that HaAm may drink. Moshe did so in the sight of the Ziknei Yisroel. And he called the shem of the place Massah (Testing), and Merivah (Dissatisfaction), because of the riv (chiding, quarreling) of the Bnei Yisroel, and because they put Hashem to the test, saying, Is Hashem among us, or not?