Sahih Muslim Book 20, Hadith Number 4705.

Chapter : About the martyrs.

It has been narrated on the authority of Abu Huraira that the, Messenger of Allah (may peace be upon him) said: While a man walks along a path, finds a thorny twig lying on the way and puts it aside, Allah would appreciate it and forgive him The Holy Prophet (may peace be upon him) said: The martyrs are of five kinds: one who dies of plague; one who dies of diarrhoea (or cholera); one who is drowned; one who is buried under debris and one who dies fighting in the way of Allah.