Musnad Abu Ya'la al-Mawsili 152:96

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from Ibrahim al-Taymi, from al-Harith ibn Suwayd, who said: Abdullah said: I entered upon the Messenger of Allah (may Allah bless him and grant him peace) while he was suffering from a fever. I touched him with my hand and said: "O Messenger of Allah, you are suffering from a severe fever." The Messenger of Allah (may Allah bless him and grant him peace) said: "Yes, indeed, I suffer from a fever as two men among you suffer." I said: "Is that because you have a double reward?" The Messenger of Allah (may Allah bless him and grant him peace) said: "Yes." Then the Messenger of Allah (may Allah bless him and grant him peace) said: "There is no Muslim who is afflicted with harm from an illness or anything else, but that Allah removes his sins thereby, just as a tree sheds its leaves."