Musnad Ahmad 30:973

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "What do you consider the martyr among you to be?" They said: "He who is killed in the way of Allah." He said: "Indeed, the martyrs of my nation are few if it is only those killed in the way of Allah. Being killed in the way of Allah is martyrdom, death by a stomach ailment is martyrdom, drowning is martyrdom, the woman who dies in childbirth is martyrdom, and the plague is martyrdom."