Sahih Muslim Book - 4, Hadith - 1062

حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ حَدَّثَنَا زُهَيْرٌ حَدَّثَنَا أَبُو إِسْحَاقَ ح قَالَ وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا أَبُو خَيْثَمَةَ عَنْ أَبِى إِسْحَاقَ عَنْ عَبْدِ اللَّهِ بْنِ يَزِيدَ قَالَ حَدَّثَنِى الْبَرَاءُ وَهُوَ غَيْرُ كَذُوبٍ أَنَّهُمْ كَانُوا يُصَلُّونَ خَلْفَ رَسُولِ اللَّهِ -صلى الله عليه وسلم- فَإِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ لَمْ أَرَ أَحَدًا يَحْنِى ظَهْرَهُ حَتَّى يَضَعَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- جَبْهَتَهُ عَلَى الأَرْضِ ثُمَّ يَخِرُّ مَنْ وَرَاءَهُ سُجَّدًا.

It was narrated that 'Abdullah bin Yazid said: "Al-Bara', who is not a liar, told me that they used to offer prayers behind the Messenger of Allah (s.a.w), and when he raised his head from bowing, I would not see anyone bending his back until the Messenger of Allah (s.a.w) had placed his forehead on the ground, then those who were behind him would go down in prostration."