Sahih Al Bukhari (549)

عن عائشة أم المؤمنين أنها قالت {صلى رسول الله صلى الله عليه وسلم في بيته وهو شاك فصلى جالسا وصلى وراءه قوم قياما فأشار إليهم أن اجلسوا فلما انصرف قال إنما جعل الإمام ليؤتم به فإذا ركع فاركعوا وإذا رفع فارفعوا وإذا صلى جالسا فصلوا جلوسا}

`A'ishah (may Allah be pleased with her), the mother of the believers narrated: Allah’s Messenger (peace be upon him) during his illness prayed at his house while sitting whereas some people prayed behind him standing. The Prophet (peace be upon him) beckoned them to sit down. On completion of the prayer, he said, ‘The Imam is to be followed: bow when he bows, raise up your heads (stand erect) when he raises his head and when he says, ‘Sami‘ a-l-lahu liman-hamidah’ (Allah heard those who sent praises to Him) say then ‘Rabbana wa laka-l-hamd’ (O our Lord! All the praises are for You), and if he prays sitting then pray sitting."