It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: Being killed in jihad in the way of Allah is martyrdom, dying from a stomach illness is also martyrdom, dying due to plague is also martyrdom, and dying in the path of Allah is also martyrdom.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10762