Prayers (Salat)
Bukhari :: Book 1 :: Volume 8 :: Hadith 425
Narrated 'Abdullah bin 'Umar:
Allah's Apostle said, "Do not enter (the places) of these people where Allah's punishment had fallen unless you do so weeping. If you do not weep, do not enter (the places of these people) because Allah's curse and punishment which fell upon them may fall upon you."