Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) came out to us and said: "Whom do you count as a martyr among you?" We said: One who is killed in the path of Allah. He (peace and blessings be upon him) said: "Whoever is killed in the path of Allah is a martyr, whoever drowns in the path of Allah is a martyr, whoever is killed by a disease of the stomach is a martyr, and the woman who dies because of her postnatal bleeding (nifas) is also a martyr."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13663