Musnad Ahmad 1012:19

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Abu Bakr ibn Hafs narrated to me, from Ibn al-Musabbah or Abu al-Musabbah, from Ibn al-Samt, from 'Ubadah ibn al-Samit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) visited 'Abdullah ibn Rawahah, and he did not move from his bed for him. He said: "Who are the martyrs of my nation?" They said: "The killing of a Muslim is martyrdom." He said: "Indeed, the martyrs of my nation are few. The killing of a Muslim is martyrdom, the plague is martyrdom, [dying from] a stomach ailment is martyrdom, drowning is martyrdom, and the woman who is killed by her child [during childbirth] is a martyr."