Riyad as-Salihin, Book 1, Hadith 38 (In-Book) ⓘ

Ibn Mas'ud (May Allah be pleased with him) reported: I visited the Prophet when he was suffering fever. I said, "You seem to be suffering greatly, O Messenger of Allah." The Prophet 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)‏‏)‏ ‏.‏