Musnad Ahmad 937:179

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman, he said: I heard Ibrahim al-Taymi narrating from his father, from Abu Dharr, from the Prophet, may Allah bless him and grant him peace: That he asked him about the first mosque placed for the people. He said: "The Sacred Mosque (al-Masjid al-Haram), then the Farthest Mosque (Bayt al-Maqdis)." He was asked: "How long was between them?" He said: "Forty years. And wherever the prayer overtakes you, then pray, for it is a mosque."