Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 82
Narrated Abu Huraira:
Allah's Apostle said, "Five are regarded as martyrs: They are those who die because of plague, abdominal disease, drowning or a falling building etc., and the martyrs in Allah's Cause."