Sahih Muslim Book 35, Hadith Number 6528.
Chapter : Excellence of remembering Allah in a low voice.
Abu Musa reported that he (and his other companions) were climbing upon the hillock along with Allah’s Messenger (may peace be upon him) and when any person climbed up, he pronounced (loudly): “There is no god but Allah, Allah is the Greatest.” Thereupon Allah’s Apostle (may peace be upon him) said: Verily, you are not supplicating One Who is deaf or absent. He said: Abu Musa or Abdullah b Qais, should I not direct you to the words (which form) the treasure of Paradise? I said: Allah’s Messenger, what are these? He said: “There is no might and no power but that of Allah.”