Narrated 'Abdullah bin Mas'ud:

I visited Allah's Apostle while he was suffering from a high fever. I touched him with my hand and said, "O Allah's Apostle! You have a high fever." Allah's Apostle ﷺ said, "Yes, I have as much fever as two men of you have." I said, "Is it because you will get a double reward?" Allah's Apostle ﷺ said, "Yes, no Muslim is afflicted with harm because of sickness or some other inconvenience, but that Allah will remove his sins for him as a tree sheds its leaves."

حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ التَّيْمِيِّ، عَنِ الْحَارِثِ بْنِ سُوَيْدٍ، قَالَ قَالَ عَبْدُ اللَّهِ بْنُ مَسْعُودٍ دَخَلْتُ عَلَى رَسُولِ اللَّهِ ﷺ وَهْوَ يُوعَكُ فَمَسِسْتُهُ بِيَدِي فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّكَ تُوعَكُ وَعْكًا شَدِيدًا. فَقَالَ رَسُولُ اللَّهِ ﷺ " أَجَلْ إِنِّي أُوعَكُ كَمَا يُوعَكُ رَجُلاَنِ مِنْكُمْ ". فَقُلْتُ ذَلِكَ أَنَّ لَكَ أَجْرَيْنِ. فَقَالَ رَسُولُ اللَّهِ ﷺ " أَجَلْ ". ثُمَّ قَالَ رَسُولُ اللَّهِ ﷺ " مَا مِنْ مُسْلِمٍ يُصِيبُهُ أَذًى مَرَضٌ فَمَا سِوَاهُ إِلاَّ حَطَّ اللَّهُ لَهُ سَيِّئَاتِهِ كَمَا تَحُطُّ الشَّجَرَةُ وَرَقَهَا ".