Musannaf Ibn Abi Shaybah

Hazrat Ubadah bin Samit (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) asked the companions, "Whom do you consider a martyr?" They replied, "The one who fights in the way of Allah and gives his life." The Prophet (peace be upon him) said, "If it were so, then there would be very few martyrs in my Ummah. The one who dies in the way of Allah is a martyr, the one who dies of plague is a martyr, the one who dies of a stomach illness is a martyr, and the woman who dies while giving birth to a child is also a martyr."