Sahih Muslim Book - 4, Hadith - 921

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى وَقُتَيْبَةُ بْنُ سَعِيدٍ وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَعَمْرٌو النَّاقِدُ وَزُهَيْرُ بْنُ حَرْبٍ وَأَبُو كُرَيْبٍ جَمِيعًا عَنْ سُفْيَانَ قَالَ أَبُو بَكْرٍ حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ عَنْ الزُّهْرِيِّ قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ يَقُولُ سَقَطَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْ فَرَسٍ فَجُحِشَ شِقُّهُ الْأَيْمَنُ فَدَخَلْنَا عَلَيْهِ نَعُودُهُ فَحَضَرَتْ الصَّلَاةُ فَصَلَّى بِنَا قَاعِدًا فَصَلَّيْنَا وَرَاءَهُ قُعُودًا فَلَمَّا قَضَى الصَّلَاةَ قَالَ إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ وَإِذَا صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا أَجْمَعُونَ.

It was narrated that Az-Zuhri said: "I heard Anas bin Malik say: 'The Prophet (s.a.w) fell from a horse and was injured on his right side. We entered upon him to visit him, and the time for prayer became due, so he led us in prayer sitting, and we offered prayer behind him sitting. When he had finished the prayer, he said: "The Imam has been appointed to be followed, so when he says the Takbir, then say the Takbir; when he prostrates, then prostrate; when he rises, then rise, and when he says: 'Sami'a Allahu liman hamidah,' then say: 'Rabbana wa lakal-hamd.' And if he prays sitting, then you should all pray sitting."