Musnad Abu Ya'la al-Mawsili 42:56

Abu Bakr narrated to us, Hatim ibn Isma'il narrated to us, from Humayd ibn Sakhr, from Abu Salamah, from Abu Sa'id, who said: I entered upon the Prophet, may Allah bless him and grant him peace, and I asked him about the mosque which was founded on righteousness. He took a handful of pebbles, then struck the ground with them, then said: "This," meaning the mosque of Medina.