Sunan Ibn Majah, Book 5, Hadith 921 (In-Book) ⓘ

It was narrated that Rashid said: “I heard Wabisah bin Ma’bad saying: ‘I saw the Messenger of Allah performing prayer, and when he bowed he made his back so straight that if water were poured on it, it would have stayed there.’”

حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُحَمَّدِ بْنِ يُوسُفَ الْفِرْيَابِيُّ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عُثْمَانَ بْنِ عَطَاءٍ، حَدَّثَنَا طَلْحَةُ بْنُ زَيْدٍ، عَنْ رَاشِدٍ، قَالَ سَمِعْتُ وَابِصَةَ بْنَ مَعْبَدٍ، يَقُولُ رَأَيْتُ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ يُصَلِّي فَكَانَ إِذَا رَكَعَ سَوَّى ظَهْرَهُ حَتَّى لَوْ صُبَّ عَلَيْهِ الْمَاءُ لاَسْتَقَرَّ ‏.‏