Sunnah.com reference : Book 2, Hadith 310English translation : Book 2, Hadith 401Arabic reference : Book 2, Hadith 406
Narrated Abu Hurairah (RA) :
Allah's Messenger (ﷺ) said: "The Imam has been appointed to be followed (in the congregational prayers). So, when he utters the Takbir, utter the Takbir too, and do not utter the Takbir until he utters it. When he bows, you should bow, and do not bow until he bows. When he says, "Allah listens to him who praises him," say, "O Allah, our Rabb, to you is the praise." When he prostrates, you prostrate; and do not prostrate until he prostrates. If he prays standing, pray standing, and if he prays sitting, all of you pray sitting."

[Reported by Abu Dawud and the wording is his].
وَعَنْ أَبِي هُرَيْرَةَ ‏- رضى الله عنه ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ إِنَّمَا جُعِلَ اَلْإِمَامُ لِيُؤْتَمَّ بِهِ, فَإِذَا كَبَّرَ فَكَبِّرُوا, وَلَا تُكَبِّرُوا حَتَّى يُكَبِّرَ, وَإِذَا رَكَعَ فَارْكَعُوا, وَلَا تَرْكَعُوا حَتَّى يَرْكَعَ, وَإِذَا قَالَ سَمِعَ اَللَّهُ لِمَنْ حَمِدَهُ, فَقُولُوا: اَللَّهُمَّ رَبَّنَا لَكَ اَلْحَمْدُ, وَإِذَا سَجَدَ فَاسْجُدُوا, وَلَا تَسْجُدُوا حَتَّى يَسْجُدَ, وَإِذَا صَلَّى قَائِمًا فَصَلُّوا قِيَامًا, وَإِذَا صَلَّى قَاعِدًا فَصَلُّوا قُعُودًا أَجْمَعِينَ }  رَوَاهُ أَبُو دَاوُدَ, وَهَذَا لَفْظُه ُ  1‏ .‏‏1 ‏- صحيح.‏ رواه أبو داود (603)‏.‏