Sahih Al Bukhari – Book of Characteristics Of Prayer
Volumn 001, Book 012, Hadith Number 699. 
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Narated By Anas bin Malik Al-Ansari : Allah's Apostle rode a horse and fell down and the right side of his body was injured. On that day he prayed one of the prayers sitting and we also prayed behind him sitting. When the Prophet finished the prayer with Taslim, he said, "The Imam is to be followed and if he prays standing then pray standing, and bow when he bows, and raise your heads when he raises his head; prostrate when he prostrates; and if he says 'Sami'a-l-lahu Liman hamida', you should say, 'Rabbana wa-laka-l hamd.'" 

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