The necessity of saying the Takbir, i.e., Allahu Akbar (Allah is the Most Great) and the commencement of As-Salat (the prayer)
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا لَيْثٌ، عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، أَنَّهُ قَالَ خَرَّ رَسُولُ اللَّهِ صلى الله عليه وسلم 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 ".
Narrated Anas bin Malik: Allah's Messenger (ﷺ) 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 hamidah say, 'Rabbana laka l-hamd', and prostrate when he prostrates."