Musnad Ahmad Hadith 12074

It is narrated from Anas bin Malik (may Allah be pleased with him) that once the Prophet (ﷺ) fell from a horse, which caused an injury to his right side. We went to visit the Prophet (ﷺ) to inquire about his health. During this, the time for prayer came, so the Prophet (ﷺ) led the prayer while sitting, and we also performed the prayer sitting. After finishing the prayer, the Prophet (ﷺ) said: "The Imam is appointed only to 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, then all of you should also perform the prayer sitting."