Sunan Ibn Majah, Volume 2, Book 5, Hadith 871 (DarusSalam) ⓘ

It was narrated that ‘Ali bin Shaiban who was part of a delegation (to the Prophet) said: “We set out until we came to the Messenger of Allah, and we gave him our oath of allegiance and performed prayer behind him. He glanced out of the corner of his eye at a man who was not settling his spine when he bowed and prostrated. When the Prophet finished the prayer, he said: ‘O Muslims, there is no prayer for the one who does not settle his spine when bowing and prostrating.’”

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُلاَزِمُ بْنُ عَمْرٍو، عَنْ عَبْدِ اللَّهِ بْنِ بَدْرٍ، أَخْبَرَنِي عَبْدُ الرَّحْمَنِ بْنُ عَلِيِّ بْنِ شَيْبَانَ، عَنْ أَبِيهِ، ‏.‏ عَلِيِّ بْنِ شَيْبَانَ - وَكَانَ مِنَ الْوَفْدِ - قَالَ خَرَجْنَا حَتَّى قَدِمْنَا عَلَى رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ فَبَايَعْنَاهُ وَصَلَّيْنَا خَلْفَهُ فَلَمَحَ بِمُؤْخِرِ عَيْنِهِ رَجُلاً لاَ يُقِيمُ صَلاَتَهُ - يَعْنِي صُلْبَهُ - فِي الرُّكُوعِ وَالسُّجُودِ فَلَمَّا قَضَى النَّبِيُّ ـ صلى الله عليه وسلم ـ الصَّلاَةَ قَالَ ‏ "‏ يَا مَعْشَرَ الْمُسْلِمِينَ لاَ صَلاَةَ لِمَنْ لاَ يُقِيمُ صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ ‏" ‏ ‏.‏