Musannaf Ibn Abi Shayba 3:5149

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