Sahih al-Bukhari, Volume 7, Book 70, Hadith 571 (USC-MSA) ⓘ

Narrated Ibn Mas`ud: I visited the Prophet while he was having a high fever. I touched him an said, "You have a very high fever" He said, "Yes, as much fever as two me of you may have." I said. "you will have a double reward?" He said, "Yes No Muslim is afflicted with hurt caused by disease or some other inconvenience, but that Allah will remove his sins as a tree sheds its leaves."

حَدَّثَنَا مُوسَى، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ مُسْلِمٍ، حَدَّثَنَا سُلَيْمَانُ، عَنْ إِبْرَاهِيمَ التَّيْمِيِّ، عَنِ الْحَارِثِ بْنِ سُوَيْدٍ، عَنِ ابْنِ مَسْعُودٍ ـ رضى الله عنه ـ قَالَ دَخَلْتُ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهْوَ يُوعَكُ فَمَسِسْتُهُ فَقُلْتُ إِنَّكَ لَتُوعَكُ وَعْكًا شَدِيدًا‏.‏ قَالَ ‏"‏ أَجَلْ كَمَا يُوعَكُ رَجُلاَنِ مِنْكُمْ ‏"‏‏.‏ قَالَ لَكَ أَجْرَانِ قَالَ ‏"‏ نَعَمْ مَا مِنْ مُسْلِمٍ يُصِيبُهُ أَذًى مَرَضٌ فَمَا سِوَاهُ إِلاَّ حَطَّ اللَّهُ سَيِّئَاتِهِ كَمَا تَحُطُّ الشَّجَرَةُ وَرَقَهَا ‏"‏‏.‏