Musnad Ahmad 548:1

'Affan narrated to us, he said: Shu'bah narrated to us, he said: Abu Bakr ibn Hafs informed me, he said: I heard Abu Musabbih, or Ibn Musabbih—Abu Bakr was uncertain—from Ibn al-Samt, from 'Ubadah ibn al-Samit: That the Messenger of Allah (peace and blessings of Allah be upon him) visited 'Abdullah ibn Rawahah, and he did not move aside for him from his bed. He said: "Do you know who the martyrs of my nation are?" 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, and a woman killed by her child [during childbirth] is martyrdom."