Sahih Muslim Book 35, Hadith Number 6526.
Abu Musa reported: We were along with Allah’s Apostle (may peace be upon him) on a journey when the people began to pronounce Allah-o-Akbar in a loud voice. Thereupon Allah’s Messenger (may peace be upon him) said: O people, show mercy to yourselves for you are not calling One Who is deaf or absent. Verily, you are calling One Who is All-Hearing (and) Near to you and is with you. Abu Musa told that he had been behind him (the Holy Prophet) and reciting: “There is neither might nor power but that of Allah.” He (the Holy Prophet), while addressing ‘Abdullah b. Qais, said: Should I not direct you to a treasure from amongst the treasurers of Paradise? I (‘Abdullah b. Qais) said: Allah’s Messenger, do it, of course. Thereupon he (the Holy Prophet) said: Then recite: “There is no might and no power but that of Allah.”