Musnad Ahmad 544:3

Abu al-Yaman narrated to us, he said: Isma'il ibn 'Ayyash narrated to us, from Muhammad ibn Ziyad al-Alhani, who said: The martyrs were mentioned in the presence of Abu 'Anabah al-Khawlani. They mentioned the one who dies of a stomach ailment, the one who dies of the plague, and the woman who dies in childbirth. Abu 'Anabah became angry and said: The companions of our Prophet narrated to us from our Prophet, may Allah bless him and grant him peace, that he said: "Indeed, the martyrs of Allah on the earth are the trustees of Allah on the earth from among His creation, whether they were killed or died."