Musnad Ahmad Hadith 9329

It is narrated from Abu Huraira (may Allah be pleased with him) that the Noble Prophet (ﷺ) said: The Imam is appointed so that he may be followed, so when he says the takbeer, you should also say the takbeer; when he bows (in ruku), you should also bow; when he says "Sami' Allahu liman hamidah," you should say "Rabbana lakal hamd"; and when he performs the prayer sitting, then all of you should also perform the prayer sitting.