Even the slightest of pain expiates sins like leaves fall off of a tree

I entered upon the Messenger of Allah ﷺ while he was suffering from a high fever. I said, "O Messenger of Allah ﷺ, you have a very 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 the prick of a thorn or something greater except that Allah expiates his sins because of it just as a tree sheds its leaves."  

دَخَلْتُ عَلَى رَسُولِ اللَّهِ ﷺ وَهْوَ يُوعَكُ، فَقُلْتُ: يَا رَسُولَ اللَّهِ، إِنَّكَ تُوعَكُ وَعْكًا شَدِيدًا! قَالَ: «أَجَلْ، إِنِّي أُوعَكُ كَمَا يُوعَكُ رَجُلاَنِ مِنْكُمْ۔» قُلْتُ: ذَلِكَ أَنَّ لَكَ أَجْرَيْنِ؟

قَالَ: «أَجَلْ، ذَلِكَ كَذَلِكَ: مَا مِنْ مُسْلِمٍ يُصِيبُهُ أَذًى شَوْكَةٌ فَمَا فَوْقَهَا إِلاَّ كَفَّرَ اللَّهُ بِهَا سَيِّئَاتِهِ كَمَا تَحُطُّ الشَّجَرَةُ وَرَقَهَا۔»