كتاب الصلاة
4  
The Book of Prayers
(21)
Chapter: If The Imam Experiences An Excuse, From Illness, Or Travelling, Etc. He May Appoint Someone Else To Lead The People In Prayer; The One Who Offers Prayer Behind The Imam Sitting Because He Is Unable To Stand Must Stand If He Is Able To Do So; And The Abrogation Of Sitting Behind A Sitting Imam For Those Who Are Able To Stand
(21)
باب اسْتِخْلاَفِ الإِمَامِ إِذَا عَرَضَ لَهُ عُذْرٌ مِنْ مَرَضٍ وَسَفَرٍ وَغَيْرِهِمَا مَنْ يُصَلِّي بِالنَّاسِ وَأَنَّ مَنْ صَلَّى خَلْفَ إِمَامٍ جَالِسٍ لِعَجْزِهِ عَنِ الْقِيَامِ لَزِمَهُ الْقِيَامُ إِذَا قَدَرَ عَلَيْهِ وَنَسْخِ الْقُعُودِ خَلْفَ الْقَاعِدِ فِي حَقِّ مَنْ قَدَرَ عَلَى الْقِيَامِ
Sahih Muslim 418 b

'A'isha reported:

It was in the house ofMaimuna that the Messenger of Allah (ﷺ) first fell ill. He asked permission from his wives to stay in her ('A'isha's) house during his illness. They granted him permission. She ('A'isha) narrated: He (the Holy Prophet) went out (for prayer) with his hand over al-Fadl b. 'Abbas and on the other hand there was another person and (due to weakness) his feet dragged on the earth. 'Ubaidullah said: I narrated this hadith to the son of 'Abbas ('Abdullah b. 'Abbas) and he said: Do you know who the man was whose name 'A'isha did not mention? It was 'Ali.
حَدَّثَنَا مُحَمَّدُ بْنُ رَافِعٍ، وَعَبْدُ بْنُ حُمَيْدٍ، - وَاللَّفْظُ لاِبْنِ رَافِعٍ - قَالاَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، قَالَ قَالَ الزُّهْرِيُّ وَأَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، أَنَّ عَائِشَةَ، أَخْبَرَتْهُ قَالَتْ، أَوَّلُ مَا اشْتَكَى رَسُولُ اللَّهِ صلى الله عليه وسلم فِي بَيْتِ مَيْمُونَةَ فَاسْـتَأْذَنَ أَزْوَاجَهُ أَنْ يُمَرَّضَ فِي بَيْتِهَا وَأَذِنَّ لَهُ - قَالَتْ - فَخَرَجَ وَيَدٌ لَهُ عَلَى الْفَضْلِ بْنِ عَبَّاسٍ وَيَدٌ لَهُ عَلَى رَجُلٍ آخَرَ وَهُوَ يَخُطُّ بِرِجْلَيْهِ فِي الأَرْضِ ‏.‏ فَقَالَ عُبَيْدُ اللَّهِ فَحَدَّثْتُ بِهِ ابْنَ عَبَّاسٍ فَقَالَ أَتَدْرِي مَنِ الرَّجُلُ الَّذِي لَمْ تُسَمِّ عَائِشَةُ هُوَ عَلِيٌّ ‏.‏
Reference : Sahih Muslim 418b
In-book reference : Book 4, Hadith 99
USC-MSA web (English) reference : Book 4, Hadith 833
  (deprecated numbering scheme)