I visited the Prophet ﷺ during his illness and touched him while he was having a fever. I said to him, "You have a high fever; is it because you will get a double reward?" He said, "Yes. No Muslim is afflicted with any harm, but that his sins will be annulled as the leave of a tree fall down."  

أَتَيْتُ النَّبِيَّ ﷺ فِي مَرَضِهِ فَمَسِسْتُهُ وَهْوَ يُوعَكُ وَعْكًا شَدِيدًا فَقُلْتُ إِنَّكَ لَتُوعَكُ وَعْكًا شَدِيدًا وَذَلِكَ أَنَّ لَكَ أَجْرَيْنِ قَالَ أَجَلْ وَمَا مِنْ مُسْلِمٍ يُصِيبُهُ أَذًى إِلاَّ حَاتَّتْ عَنْهُ خَطَايَاهُ كَمَا تَحَاتُّ وَرَقُ الشَّجَرِ  

Messenger of Allah ﷺ entered upon sick man to pay him a visit, and said to him, "Don't worry, Allah willing, (your sickness will be) an expiation for your sins." The man said, "No, it is but a fever that is boiling within an old man and will send him to his grave." On that, the Prophet ﷺ said, "Then yes, it is so."  

أَنَّ رَسُولَ اللَّهِ ﷺ دَخَلَ عَلَى رَجُلٍ يَعُودُهُ فَقَالَ لاَ بَأْسَ طَهُورٌ إِنْ شَاءَ اللَّهُ فَقَالَ كَلاَّ بَلْ حُمَّى تَفُورُ عَلَى شَيْخٍ كَبِيرٍ كَيْمَا تُزِيرَهُ الْقُبُورَ قَالَ النَّبِيُّ ﷺ فَنَعَمْ إِذًا