Musannaf Ibn Abi Shayba 11:171

19473. Abdullah bin Numayr narrated to us, Muhammad bin Ishaq narrated to us, from Abu Malik bin Tha'labah, from Umar bin al-Hakam bin Thawban, from Abu Hurayrah, who said: I heard the Messenger of Allah (ﷺ) say: "What do you consider the martyr?" He said: They said, "The one killed in the way of Allah." He said: "Then the martyrs of my nation would be few. The one killed in the way of Allah is a martyr, and the one who falls from his mount in the way of Allah is a martyr, and the one who drowns in the way of Allah is a martyr, and the one who dies of pleurisy in the way of Allah is a martyr"—meaning an ulcer of the side.