Musnad Ahmad 1012:118

Abdullah narrated to us, Abu Bahr Abd al-Wahid ibn Ghiyath narrated to us, Hammad ibn Salamah narrated to us, from Abu Sinan, from Ya'la ibn Shaddad, who said: I heard Ubadah ibn al-Samit saying: The Messenger of Allah (peace and blessings of Allah be upon him) visited me among a group of his companions and said: "Do you know who the martyrs of my nation are?" (He said it) twice or three times. They remained silent. Ubadah said: Inform us, O Messenger of Allah. He said: "The one killed in the way of Allah is a martyr, the one who dies of a stomach ailment is a martyr, the one who dies of the plague is a martyr, and the woman who dies in childbirth is a martyr; her child will drag her by his umbilical cord to Paradise."