Yahya related to me from Malik from Ibn Shihab from Anas ibnMalik that the Messenger of Allah, may Allah bless him and grant himpeace, fell off his horse when riding, and his right side was scraped,so he did one of the prayers sitting, and we prayed behind himsitting. When he left, he said, "The imam is appointed to be followed.If he prays standing, then pray standing, and when he goes into ruku,go into ruku, and when he rises, rise, and when he says, 'Allah hearswhoever praises him,' say, 'Our Lord, praise belongs to You,' and ifhe prays sitting, then all of you pray sitting."
App reference: Book 8, Hadith 17
USC-MSA web (English) reference: Book 8, Hadith 17
Arabic reference: Book 8, Hadith 306