Abu Dawud Book 8, Hadith Number 2493.

Narated By AbuMalik al-Ash’ari : AbuMalik heard the Apostle of Allah (PBUH) say: He who goes forth in Allah’s path and dies or is killed is a martyr, or has his neck broken through being thrown by his horse or by his camel, or is stung by a poisonous creature, or dies on his bed by any kind of death Allah wishes is a martyr and will go to Paradise.