Musnad Ahmad 1012:20

Waki’ narrated to us, he said: Hisham ibn al-Ghazi narrated to us, from ‘Ubadah ibn Nasi, from ‘Ubadah ibn al-Samit, that the Prophet, peace and blessings of Allah be upon him, said: "What do you consider the martyr among you to be?" They said: The one who fights and is killed in the way of Allah. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, the martyrs of my nation are few. The one killed in the way of Allah is a martyr, the one who dies of the plague is a martyr, the one who dies of a stomach ailment is a martyr, and the woman who dies in childbirth is a martyr"—meaning the one who has just given birth.