Musnad Ahmad 26:795

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from Ibrahim al-Taymi, from al-Harith ibn Suwayd, who said: Abdullah said: I entered upon the Prophet, may Allah bless him and grant him peace, while he was suffering from a fever. I placed my hand upon him and said: "You are suffering from a severe fever." He said: "I suffer from a fever as two men among you suffer." I said: "Is that because you have a double reward?" He said: "Yes. There is no believer who is afflicted by an illness or anything else, but that Allah removes his sins thereby, just as a tree sheds its leaves."