Musnad Ahmad 30:1186

Rawh narrated to us, Malik ibn Anas narrated to us, from Sumayy, the freed slave of Abu Bakr ibn Abd al-Rahman, from Abu Salih al-Samman, from Abu Hurairah, that the Prophet (peace and blessings of Allah be upon him) said: "The martyrs are five: the one who dies of the plague, the one who dies of a stomach ailment, the one who drowns, the one who dies under a collapsed building, and the martyr in the way of Allah, the Almighty and Majestic."