Musnad Ahmad Hadith 11187

Abu Salamah bin Abdur Rahman (may Allah have mercy on him) says that once Abdur Rahman bin Abi Sa’d (may Allah be pleased with him) passed by me, and I asked him, "What have you heard from your father about this mosque whose foundation was laid on piety?" He said that his father had said: Once I went to one of the houses of the Prophet (peace be upon him) and asked the Prophet (peace be upon him), "O Messenger of Allah (peace be upon him), which is the mosque whose foundation was laid on piety?" The Prophet (peace be upon him) took a handful of pebbles and struck them on the ground and said, "It is this mosque of the Prophet in Madinah Munawwarah." After hearing this hadith, Abdur Rahman (may Allah be pleased with him) said, "I bear witness that I have also heard your father mention it in the same way."