Sahih Bukhari Volume 004, Book 055, Hadith Number 585.
Narated By Abu Dhar : I said, "O Allah's Apostle! Which mosque was first built on the surface of the earth?" He said, "Al-Masjid-ul-,Haram (in Mecca)." I said, "Which was built next?" He replied "The mosque of Al-Aqsa (in Jerusalem)." I said, "What was the period of construction between the two?" He said, "Forty years." He added, "Wherever (you may be, and) the prayer time becomes due, perform the prayer there, for the best thing is to do so (i.e. to offer the prayers in time)."