The necessity of saying the Takbir, i.e., Allahu Akbar (Allah is the Most Great) and the commencement of As-Salat (the prayer)

Narrated Anas bin Malik Al-Ansari:

Allah's Apostle 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 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 hamida", 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 ﷺ 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 ".

Narrated Anas bin Malik:

Allah's Apostle fell from a horse and got injured so he led the prayer sitting and we also prayed sitting. When he completed the prayer he said, "The Imam is to be followed;
if he says Takbir then say Takbir, bow if he bows;
raise your heads when he raises his head, when he says, 'Sami' a-l-lahu Liman hamida say, 'Rabbana laka-l-hamd', and prostrate when he prostrates."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا لَيْثٌ، عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّهُ قَالَ خَرَّ رَسُولُ اللَّهِ ﷺ عَنْ فَرَسٍ فَجُحِشَ فَصَلَّى لَنَا قَاعِدًا فَصَلَّيْنَا مَعَهُ قُعُودًا، ثُمَّ انْصَرَفَ فَقَالَ " إِنَّمَا الإِمَامُ ـ أَوْ إِنَّمَا جُعِلَ الإِمَامُ ـ لِيُؤْتَمَّ بِهِ، فَإِذَا كَبَّرَ فَكَبِّرُوا، وَإِذَا رَكَعَ فَارْكَعُوا، وَإِذَا رَفَعَ فَارْفَعُوا، وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ. فَقُولُوا رَبَّنَا لَكَ الْحَمْدُ. وَإِذَا سَجَدَ فَاسْجُدُوا ".