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 ‏"‏ ‏.‏

Anas reported: The Messenger of Allah (PBUH) 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." Translation Not Available

Book Ref: Sahih Muslim Book 4 Hadith 820
Web Ref:  Sahih Muslim Book 4 Hadith 411 d