Majma al-Zawaid 9560 — The means by which martyrdom is attained

Muhammad bin Ziyad al-Alhani narrates: It was mentioned in the presence of Abu Inabah al-Khawlani that among the martyrs are the one who dies of plague, the one who dies of an abdominal illness, and the woman who dies during postnatal bleeding. Abu Inabah became angry and said: The Companions of our Prophet narrated this hadith to us from our Prophet, that he said: "The martyrs of Allah on earth are Allah's trustees over His creation, whether they die by being killed or die a natural death."
Ahmad recorded it, and its narrators are thiqat (trustworthy).