Hadith 8156

وَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ : " إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ ، فَلَا تَخْتَلِفُوا عَلَيْهِ ، وَإِذَا كَبَّرَ فَكَبِّرُوا ، وَإِذَا رَكَعَ فَارْكَعُوا ، وَإِذَا قَالَ : سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ ، فَقُولُوا : اللَّهُمَّ رَبَّنَا لَكَ الْحَمْدُ ، وَإِذَا سَجَدَ فَاسْجُدُوا ، وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا أَجْمَعِينَ " .
And the Prophet (peace and blessings be upon him) said: The Imam is appointed only so that he may be followed, so do not differ from the Imam. When he says the takbeer, you also say the takbeer. When he bows (in ruku'), you also bow. When he says "Sami' Allahu liman hamidah," you say "Allahumma Rabbana lakal hamd." When he prostrates, you also prostrate. And when he prays sitting, then all of you should also pray sitting.
Hadith Reference مسند احمد / مسند المكثرين من الصحابة / 8156
Hadith Grading حکم دارالسلام: إسناده صحيح، خ: 734، م: 414