Musnad Ahmad

It is narrated from Hazrat Anas (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) fell from a horse, which caused an injury to his right side. We went to visit the Prophet (peace and blessings be upon him), and during this time, the time for prayer arrived. The Prophet (peace and blessings be upon him) led the prayer while sitting, and we also performed the prayer sitting. After finishing the prayer, the Prophet (peace and blessings be upon him) said: "The Imam is appointed so that he may be followed, so when he says the takbeer, you also say the takbeer; when he bows (in ruku‘), you also bow; when he prostrates, you also prostrate; when he says 'Sami‘ Allahu liman hamidah,' you say 'Rabbana wa lakal hamd'; and when he performs the prayer sitting, you should all also perform the prayer sitting."