(Imām Bukhārī narrates): Ādam narrated to us, he said: Isrā'īl narrated to us, from Abī Ishāq, from 'Abdullāh ibn Yazīd al-Khatmī, he said: Al-Barā' ibn 'Āzib narrated to us and he is not a liar, he said:
We used to pray behind the Prophet ﷺ; so when he said "Sami'a Allāhu liman hamidah," none of us would bend his back until the Prophet ﷺ placed his forehead upon the ground.
حَدَّثَنَا آدَمُ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ أَبِي إِسْحَاقَ، عَنْ عَبْدِ اللَّهِ بْنِ يَزِيدَ الْخَطْمِيِّ، حَدَّثَنَا الْبَرَاءُ بْنُ عَازِبٍ ـ وَهْوَ غَيْرُ كَذُوبٍ ـ قَالَ كُنَّا نُصَلِّي خَلْفَ النَّبِيِّ صلى الله عليه وسلم فَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ. لَمْ يَحْنِ أَحَدٌ مِنَّا ظَهْرَهُ حَتَّى يَضَعَ النَّبِيُّ صلى الله عليه وسلم جَبْهَتَهُ عَلَى الأَرْضِ.
Reference: Sahih Al Bukhari 811