Mukhtasar Muslim (Abridged) 1082 — Martyrs are of five types

Narrated from Sayyiduna Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: A man was walking when he saw a thorny branch on the road, so he removed it (from the road). Because of this, Allah rewarded him by forgiving him. And the Prophet (peace and blessings of Allah be upon him) said that there are five types of martyrs: the one who dies of plague (an epidemic, in that time plague was caused by vomiting and diarrhea), the one who dies of a stomach illness (such as diarrhea, dysentery, or dropsy), the one who drowns, the one who is crushed to death, and the one who is killed in the way of Allah.