Mishkat al-Masabih 1136
Al-Bara’ b. ‘Azib said:
We used to pray behind the Prophet, and when he said, "God listens to him who praises Him," none of us bowed his back till the Prophet placed his forehead on the ground. 

(Bukhari and Muslim.)
عَنِ الْبَرَاءِ بْنِ عَازِبٍ قَالَ: كُنَّا نُصَلِّي خَلْفَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَإِذَا قَالَ: «سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ» . لَمْ يَحْنِ أَحَدٌ مِنَّا ظَهْرَهُ حَتَّى يَضَعَ النَّبِيُّ صَلَّى الله عَلَيْهِ وَسلم جَبهته على الأَرْض