(13) باب اسْتِحْبَابِ خَفْضِ الصَّوْتِ بِالذِّكْرِ
Abu Musa reported:
We were along with Allah's Apostle (ﷺ) on a journey when the people began to pronounce "Allahu Akbar" in a loud voice. Thereupon Allah's Messenger (ﷺ) 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 said that he had been behind him (the Prophet) and reciting: "There is neither might nor power but that of Allah." He (the Prophet), while addressing 'Abdullah b. Qais, said: Should I not direct you to a treasure from amongst the treasures of Paradise? I ('Abdullah b. Qais) said: Allah's Messenger, do it, of course. Thereupon he (the Prophet) said: Then recite: "There is no might and no power but that of Allah."
Reference: Sahih Muslim 2704 a
In-book reference: Book 48, Hadith 56
USC-MSA web (English) reference(deprecated numbering scheme): Book 35, Hadith 6526