Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 819
Anas b. Malik reported: The Messenger of Allah (may peace be upon him) fell down from a horse and his right side was grazed, and the rest of the hadith is the same with the addition of these words:" When he (the Imam) says prayer standing, you should also do so."