Al-Adab Al-Mufrad
It is narrated by Abdullah bin Amr that Prophet (ﷺ) said, "When a person falls ill then the reward of those deeds is also recorded for him which he used to perform when he was healthy.
حَدَّثَنَا قَبِيصَةُ بْنُ عُقْبَةَ، قَالَ: حَدَّثَنَا سُفْيَانُ، عَنْ عَلْقَمَةَ بْنِ مَرْثَدٍ، عَنِ الْقَاسِمِ بْنِ مُخَيْمِرَةَ، عَنْ عَبْدِ اللهِ بْنِ عَمْرٍو، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: مَا مِنْ أَحَدٍ يَمْرَضُ، إِلاَّ كُتِبَ لَهُ مِثْلُ مَا كَانَ يَعْمَلُ وَهُوَ صَحِيحٌ.
Reference