Sayyidah Aishah (may Allah be pleased with her) said: The Messenger of Allah (peace and blessings be upon him) fell ill, and his Companions came to visit him. The Messenger of Allah (peace and blessings be upon him) led the prayer sitting, and the Companions prayed behind him standing, so the Messenger of Allah (peace and blessings be upon him) signalled to them to sit, and they sat down. When he (peace and blessings be upon him) had finished the prayer, he said: "The imam is there only to be followed. So when he bows, bow as well; when he raises his head from bowing, raise your heads as well; and when he prays sitting, pray sitting as well."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3656