Riyad as-Salihin 38
Ibn Mas'ud (May Allah be pleased with him) reported:
I visited the Prophet (ﷺ) when he was suffering fever. I said, "You seem to be suffering greatly, O Messenger of Allah." The Prophet (ﷺ) replied, "Yes, I suffer as much as two persons." I said, "Is that because you have a double reward?" He replied that that was so and then said, "No Muslim is afflicted by a harm, be it the pricking of a thorn or something more (painful than that), but Allah thereby causes his sins to fall away just as a tree sheds its leaves". [Al-Bukhari and Muslim] .
وعن ابن مسعود رضي الله عنه قال‏:‏ دخلت على النبي صلى الله عليه وسلم وهو يوعك فقلت‏:‏ يارسول الله إنك توعك وعكاً شديداً قال‏:‏ ‏"‏أجل  إني أوعك كما يوعك رجلان منكم‏"‏ قلت‏:‏ ذلك أن لك أجرين ‏؟‏ قال‏:‏ ‏"‏أجل ذلك كذلك ما من مسلم يصيبه أذى؛  شوكة فما فوقها إلا كفر الله بها سيئاته ، وحطت عنه ذنوبه كما تحط الشجرة ورقها‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏ ‏.‏