Sahih Bukhari Volume 001, Book 011, Hadith Number 656.
Narated By 'Aisha : The mother of the believers: Allah's Apostle during his illness prayed at his house while sitting whereas some people prayed behind him standing. The Prophet 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-hamida ' (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."