Sahih Muslim 411dBook 4, Hadith 86
حَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا مَعْنُ بْنُ عِيسَى، عَنْ مَالِكِ بْنِ أَنَسٍ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَكِبَ فَرَسًا فَصُرِعَ عَنْهُ فَجُحِشَ شِقُّهُ الأَيْمَنُ . بِنَحْوِ حَدِيثِهِمْ وَفِيهِ " إِذَا صَلَّى قَائِمًا فَصَلُّوا قِيَامًا " .
The Messenger of Allah (ﷺ) rode a horse and fell down from it and his right side was grazed, and the rest of the hadith is the same, and (these words) are found in it:" When he (the Imam) says prayer in an erect posture, you should also say it in an erect posture."