Musnad Ahmad 1012:37

Surayj narrated to us, al-Mu‘afa narrated to us, Mughira ibn Ziyad narrated to us, from ‘Ubada ibn Nasi, from al-Aswad ibn Tha‘laba, from ‘Ubada ibn al-Samit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came to me while I was ill among some people of the Ansar who were visiting me. He said: "Do you know who the martyr is?" They remained silent. He said: "Do you know who the martyr is?" They remained silent. He said: "Do you know who the martyr is?" I said to my wife: "Support me," and she supported me. I said: "He who submits, then migrates, then is killed in the way of Allah, he is a martyr." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the martyrs of my nation are few. Being killed in the way of Allah is martyrdom, death by a stomach ailment is martyrdom, drowning is martyrdom, and the woman who dies in childbirth is a martyr."