Musnad Ahmad 1012:90

'Affan narrated to us, 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. He said: He did not move aside for him from his bed. He said: "Do you know who the martyrs of my nation are?" They said: Being killed as a Muslim is martyrdom. He said: "Indeed, the martyrs of my nation are then few. Being killed as a Muslim is martyrdom, the plague is martyrdom, and a woman killed by her child [while] pregnant is martyrdom."