(19) باب ائْتِمَامِ الْمَأْمُومِ بِالإِمَامِ
Anas reported:
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."
Reference: Sahih Muslim 411 d
In-book reference: Book 4, Hadith 86
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 820