Language:
كتاب الأذان
10
Call to Prayers (Adhaan)
(82)
Chapter: The necessity of saying the Takbir, i.e., Allahu Akbar (Allah is the Most Great) and the commencement of As-Salat (the prayer)
(82)
باب إِيجَابِ التَّكْبِيرِ وَافْتِتَاحِ الصَّلاَةِ
Sahih al-Bukhari 732

Narrated Anas bin Malik Al-Ansari:

Allah's Messenger (ﷺ) rode a horse and fell down and the right side of his body was injured. On that day he prayed one of the prayers sitting and we also prayed behind him sitting. When the Prophet (ﷺ) finished the prayer with Taslim, he said, "The Imam is to be followed and if he prays standing then pray standing, and bow when he bows, and raise your heads when he raises his head; prostrate when he prostrates; and if he says "Sami`a l-lahu liman hamidah", you should say, "Rabbana wa laka l-hamd.:

حَدَّثَنَا أَبُو الْيَمَانِ، قَالَ أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي أَنَسُ بْنُ مَالِكٍ الأَنْصَارِيُّ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَكِبَ فَرَسًا، فَجُحِشَ شِقُّهُ الأَيْمَنُ، قَالَ أَنَسٌ ـ رضى الله عنه ـ فَصَلَّى لَنَا يَوْمَئِذٍ صَلاَةً مِنَ الصَّلَوَاتِ وَهْوَ قَاعِدٌ، فَصَلَّيْنَا وَرَاءَهُ قُعُودًا، ثُمَّ قَالَ لَمَّا سَلَّمَ ‏ "‏ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ، فَإِذَا صَلَّى قَائِمًا فَصَلُّوا قِيَامًا، وَإِذَا رَكَعَ فَارْكَعُوا، وَإِذَا رَفَعَ فَارْفَعُوا، وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ‏.‏ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ ‏"‏‏.‏
Reference : Sahih al-Bukhari 732
In-book reference : Book 10, Hadith 126
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 699
  (deprecated numbering scheme)