Sahih Bukhari: Hadith 672

the mother of the believers: Allah's Messenger (ﷺ) during his illness prayed at his house while sitting whereas some people prayed behind him standing. The Prophet (ﷺ) 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."