Sahih Bukhari
Prophets
Volume: 004 / Book Number: 055 / Hadith Number: 636
Narrated Abu Dhaar:
I said, "O Allah's Apostle! Which mosque was built first?" He replied, "Al-Masjid-ul-Haram." I asked, "Which (was built) next?" He replied, "Al-Masjid-ul-Aqs-a (i.e. Jerusalem)." I asked, "What was the period in between them?" He replied, "Forty (years)." He then added, "Wherever the time for the prayer comes upon you, perform the prayer, for all the earth is a place of worshipping for you."