Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1056
Abu Dharr reported: I said: Messenger of Allah, which mosque was set up first on the earth? He said: Al-Masjid al-Haram (the sacred). I (again) said: Then which next? He said: It was the Masjid Aqsa. I (again) said: How long the space of time (between their setting up)? He (the Holy Prophet) said: It was forty years. And whenever the time comes for prayer, pray there, for that is a mosque; and in the hadith transmitted by Abu Kamil (the words are):" Whenever time comes for prayer, pray, for that is a mosque (for you)."