Sahih Muslim Book 35, Hadith Number 6532.
Chapter : Excellence of remembering Allah in a low voice.
Abu Musa Ash’ari reported that Allah’s Messenger (may peace be upon him) said to him: Should I not direct you to the words from the treasures of Paradise, or he said: Like a treasure from the treasures of Paradise? I said: Of course, do that. Thereupon he said: “There is no might and no power but that of Allah.”