Musnad Ahmad 30:3631

'Abd al-Samad narrated to us, Hammad narrated to us, from Suhayl, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The one killed in the way of Allah is a martyr, the one who dies of the plague is a martyr, the one who dies of a stomach ailment is a martyr, and whoever dies in the way of Allah, he is a martyr."