Musannaf Ibn Abi Shaybah

Hazrat Anas (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) fell from a horse and his right side was injured. We went to visit him, and the time for prayer arrived. The Prophet (peace and blessings be upon him) led us in prayer while sitting, and we also prayed sitting behind him. When the Prophet (peace and blessings be upon him) completed the prayer, he said that the Imam is appointed so that he may be followed. When he says the takbeer, you should also say the takbeer; when he bows (in ruku‘), you should also bow; when he says "Sami‘a Allahu liman hamidah," you should say "Rabbana wa lakal hamd." When he prays sitting, then all of you should also pray sitting.