The severity of the illness.

Yaʿqūb b. Ibrāhīm > Yaḥyá > Sufyān > Sulaymān > Ibrāhīm > al-Ḥārith b. Sūwayd > ʿAbdullāh

I visited Messenger of Allah ﷺ while he was suffering from a high fever. I said, "O Messenger of Allah ﷺ! You have a high fever." He said, "Yes, I have as much fever as two men of you." I said, "Is it because you will have a double reward?" He said, "Yes, it is so. No Muslim is afflicted with any harm, even if it were the prick of a thorn, but that Allah expiates his sins because of that, as a tree sheds its leaves."

أَخْبَرَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ قَالَ حَدَّثَنَا يَحْيَى قَالَ حَدَّثَنَا سُفْيَانُ قَالَ حَدَّثَنِي سُلَيْمَانُ عَنْ إِبْرَاهِيمَ عَنِ الْحَارِثِ بْنِ سُوَيْدٍ عَنْ عَبْدِ اللهِ قَالَ

دَخَلْتُ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يُوعِكُ وَعْكًا شَدِيدًا قُلْتُ يَا رَسُولَ اللهِ إِنَّكَ تُوعِكُ وَعْكًا شَدِيدًا ذَلِكَ بِأَنَّ لَكَ أَجْرَيْنِ قَالَ «أَجَلْ إِنَّهُ كَذَلِكَ مَا مِنْ مُسْلِمٍ يُصِيبُهُ أَذًى مِنْ مَرَضٍ فَمَا سِوَاهُ إِلَّا كَفَّرَ اللهُ بِهِ سَيِّئَاتِهِ كَمَا تَحُطُّ الشَّجَرَةُ وَرَقَهَا»