Musnad Ahmad 937:101

Waki’ narrated to us, from Sufyan, from al-A’mash, from Ibrahim al-Taymi, from his father, from Abu Dharr, who said: I said: "O Messenger of Allah, which mosque was established first?" He said: "The Sacred Mosque (al-Masjid al-Haram)." I said: "Then which?" He said: "Then the Furthest Mosque (al-Masjid al-Aqsa)." I said: "How much was between them?" He said: "Forty years. Then, wherever the prayer overtakes you, pray, for it is a mosque."