The Book of Prayers
4/21. 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
Sahih Muslim
وَحَدَّثَنِي مُحَمَّدُ بْنُ رَافِعٍ، وَعَبْدُ بْنُ حُمَيْدٍ، جَمِيعًا عَنْ عَبْدِ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي أَنَسُ بْنُ مَالِكٍ، قَالَ لَمَّا كَانَ يَوْمُ الاِثْنَيْنِ . بِنَحْوِ حَدِيثِهِمَا .
This hadith is narrated on the authority of Anas b. Malik by another chain of transmitters.
Reference : Sahih Muslim 419cIn-book reference : Book 4, Hadith 108USC-MSA web (English) reference : Book 4, Hadith 842