Musnad Ahmad 30:2567

Muhammad ibn Ubayd narrated to us, he said: Muhammad—meaning Ibn Ishaq—narrated to us, from Abu Malik ibn Tha'labah ibn Abi Malik al-Qurazi, from Umar ibn al-Hakam ibn Thawban, from Abu Hurayrah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "What do you consider the martyr?" They said: "The one who fights in the way of Allah until he is killed." He said: "Indeed, the martyr in my nation is then few. The one killed in the way of Allah is a martyr, the one who dies of the plague in the way of Allah is a martyr, the one who drowns in the way of Allah is a martyr, the one who falls from his mount in the way of Allah is a martyr, and the one who dies of pleurisy in the way of Allah is a martyr." Muhammad said: "The *majnub* is the one who has pleurisy."