Sahih Muslim Book 007, Hadith Number 3221.
Chapter : The mosque founded on piety in the mosque of the Apostle (may peace be upon him) at Medina.
Abu Salama b. Abd al-Rabman reported: 'Abd al-Rabman b. Abu Sa'id al-Khudri (Allah be pleased with him) happened to pass by me and I said to him. How did you hear your father making mention of the mosque founded on Piety? He said: My father said: I went to Allah's Messenger (may peace be upon him) as he was in the house of one of his wives, and said: Messenger of Allah, which of the two mosques is founded on piety? Thereupon he took a handful of pebbles and threw them on the ground and then said: This is the very mosque of yours (mosque at Medina). He (the narrator) said: I bear witness that I heard your father making mention of it.