Sahih Muslim Book - 4, Hadith - 928

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا لَيْثٌ ح و حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ أَخْبَرَنَا اللَّيْثُ عَنْ أَبِي الزُّبَيْرِ عَنْ جَابِرٍ قَالَ اشْتَكَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَصَلَّيْنَا وَرَاءَهُ وَهُوَ قَاعِدٌ وَأَبُو بَكْرٍ يُسْمِعُ النَّاسَ تَكْبِيرَهُ فَالْتَفَتَ إِلَيْنَا فَرَآنَا قِيَامًا فَأَشَارَ إِلَيْنَا فَقَعَدْنَا فَصَلَّيْنَا بِصَلَاتِهِ قُعُودًا فَلَمَّا سَلَّمَ قَالَ إِنْ كِدْتُمْ آنِفًا لَتَفْعَلُونَ فِعْلَ فَارِسَ وَالرُّومِ يَقُومُونَ عَلَى مُلُوكِهِمْ وَهُمْ قُعُودٌ فَلَا تَفْعَلُوا ائْتَمُّوا بِأَئِمَّتِكُمْ إِنْ صَلَّى قَائِمًا فَصَلُّوا قِيَامًا وَإِنْ صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا.

It was narrated that Jabir said: "The Messenger of Allah (s.a.w) was sick and we prayed behind him while he was (offering prayers) sitting and Abu Bakr was repeating his Takbir so that the people could hear. He turned to us and saw us standing, and gestured to us that we should sit, so we followed him in prayer, sitting down. When he had said the Salam he said: 'Just now you almost did what the Persians and Romans do; they stand for their kings when they are sitting. Do not do that. Follow your Imam: if he prays standing then pray standing, and if he prays sitting then pray sitting."'