Fighting for the Cause of Allah (Jihaad)

Volume 4, Book 52, 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."

Volume 4, Book 52, Number 82 (Source)