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 c

'A'isha, the wife of the Apostle (ﷺ), said:

When the Messenger of Allah (ﷺ) fell ill and his illness became serious, he asked permission from his wives to stay in my house during his illness. They gave him permission to do so. He stepped out (of'A'isha's apartment for prayer) supported by two persons. (He was so much weak) that his feet dragged on the ground and he was being supported by 'Abbas b. 'Abd al-Muttalib and another person. 'Ubaidullah said: I informed 'Abdullah (b. 'Abbas) about that which 'A'isha had said. 'Abdullah b. 'Abbas said: Do you know the man whose name 'A'isha did not mention? He said: No. Ibn 'Abbas said: It was 'Ali.
حَدَّثَنِي عَبْدُ الْمَلِكِ بْنُ شُعَيْبِ بْنِ اللَّيْثِ، حَدَّثَنِي أَبِي، عَنْ جَدِّي، قَالَ حَدَّثَنِي عُقَيْلُ بْنُ خَالِدٍ، قَالَ ابْنُ شِهَابٍ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ بْنِ مَسْعُودٍ، أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم قَالَتْ لَمَّا ثَقُلَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَاشْتَدَّ بِهِ وَجَعُهُ اسْتَأْذَنَ أَزْوَاجَهُ أَنْ يُمَرَّضَ فِي بَيْتِي فَأَذِنَّ لَهُ فَخَرَجَ بَيْنَ رَجُلَيْنِ تَخُطُّ رِجْلاَهُ فِي الأَرْضِ بَيْنَ عَبَّاسِ بْنِ عَبْدِ الْمُطَّلِبِ وَبَيْنَ رَجُلٍ آخَرَ ‏.‏ قَالَ عُبَيْدُ اللَّهِ فَأَخْبَرْتُ عَبْدَ اللَّهِ بِالَّذِي قَالَتْ عَائِشَةُ فَقَالَ لِي عَبْدُ اللَّهِ بْنُ عَبَّاسٍ هَلْ تَدْرِي مَنِ الرَّجُلُ الآخَرُ الَّذِي لَمْ تُسَمِّ عَائِشَةُ قَالَ قُلْتُ لاَ ‏.‏ قَالَ ابْنُ عَبَّاسٍ هُوَ عَلِيٌّ ‏.‏
Reference : Sahih Muslim 418c
In-book reference : Book 4, Hadith 100
USC-MSA web (English) reference : Book 4, Hadith 834
  (deprecated numbering scheme)