Musnad Ahmad 31:199

Yahya narrated to us, from Humayd al-Kharrat, who said: I heard Abu Salamah ibn Abd al-Rahman say: Abd al-Rahman ibn Abi Sa’id al-Khudri passed by me, and I said to him: How did you hear your father speak regarding the mosque that was founded on righteousness? He said: My father said: I entered upon the Messenger of Allah, may Allah bless him and grant him peace, in the house of one of his wives, and I said: O Messenger of Allah, which of the two mosques is the one that was founded on righteousness? He took a handful of pebbles and struck the ground with them, then said: "It is this, the mosque of Medina." He said: So I said to him: I bear witness that I heard your father mention it in this way.