Sahih Muslim 413 a
Jabir reported:
The Messenger of Allah (ﷺ) was ill and we said prayer behind him and he was sitting. And Abu Bakr was making audible to the people his takbir. As he paid his attention towards us he saw us standing and (directed us to sit down) with a gesture. So we sat down and said our prayer with his prayer in a sitting posture. After uttering salutation he said: You were at this time about to do an act like that of the Persians and the Romans. They stand before their kings while they sit, so don't do that; follow your Imams. If they say prayer standing, you should also do so, and if they say prayer sitting, you should also say prayer sitting.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ، أَخْبَرَنَا اللَّيْثُ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ اشْتَكَى رَسُولُ اللَّهِ صلى الله عليه وسلم فَصَلَّيْنَا وَرَاءَهُ وَهُوَ قَاعِدٌ وَأَبُو بَكْرٍ يُسْمِعُ النَّاسَ تَكْبِيرَهُ فَالْتَفَتَ إِلَيْنَا فَرَآنَا قِيَامًا فَأَشَارَ إِلَيْنَا فَقَعَدْنَا فَصَلَّيْنَا بِصَلاَتِهِ قُعُودًا فَلَمَّا سَلَّمَ قَالَ ‏
"‏ إِنْ كِدْتُمْ آنِفًِا لَتَفْعَلُونَ فِعْلَ فَارِسَ وَالرُّومِ يَقُومُونَ عَلَى مُلُوكِهِمْ وَهُمْ قُعُودٌ فَلاَ تَفْعَلُوا ائْتَمُّوا بِأَئِمَّتِكُمْ إِنْ صَلَّى قَائِمًا فَصَلُّوا قِيَامًا وَإِنْ صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا ‏"‏ ‏.‏