Narrated by Anas bin Malik:
Allah’s Apostle fell from a horse and got injured so he led the prayer sitting and we also prayed sitting. When he completed the prayer he said, “The Imam is to be followed; if he says Takbir then say Takbir, bow if he bows; raise your heads when he raises his head, when he says, ‘Sami’ a-l-lahu Liman hamida say, ‘Rabbana laka-l-hamd’, and prostrate when he prostrates.”
[Bukhari Vol 1 Book 12 Hadith 700]