Sunan Ibn Majah Book 5, Hadith 1240

حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ الْمِصْرِيُّ، أَنْبَأَنَا اللَّيْثُ بْنُ سَعْدٍ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ اشْتَكَى رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ فَصَلَّيْنَا وَرَاءَهُ وَهُوَ قَاعِدٌ وَأَبُو بَكْرٍ يُكَبِّرُ يُسْمِعُ النَّاسَ تَكْبِيرَهُ فَالْتَفَتَ إِلَيْنَا فَرَآنَا قِيَامًا فَأَشَارَ إِلَيْنَا فَقَعَدْنَا فَصَلَّيْنَا بِصَلاَتِهِ قُعُودًا فَلَمَّا سَلَّمَ قَالَ ‏ "‏ إِنْ كِدْتُمْ أَنْ تَفْعَلُوا فِعْلَ فَارِسَ وَالرُّومِ يَقُومُونَ عَلَى مُلُوكِهِمْ وَهُمْ قُعُودٌ فَلاَ تَفْعَلُوا ائْتَمُّوا بِأَئِمَّتِكُمْ إِنْ صَلَّى قَائِمًا فَصَلُّوا قِيَامًا وَإِنْ صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا ‏"‏ ‏.‏

It was narrated that Jabir said:“The Messenger of Allah (ﷺ) fell ill, and we prayed behind him while he was sitting down, and Abu Bakr was saying the Takbir so that the people could hear them. He turned to us and saw us standing, so he gestured to us to sit down. When he had said the Salam, he said: ‘You were about to do the action of the Persians and Romans, who remain standing while their kings are seated. Do not do that. Follow the lead of your Imam; if he prays standing, then pray standing, and if he prays sitting down, then pray sitting down.’”