Sunan an-Nasa'i, Volume 2, Book 11, Hadith 1028 (DarusSalam) ⓘ
It was narrated that Abu Mas'ud said: "The Messenger of Allah said: 'The prayer is not valid if a man does not bring his backbone to rest while bowing and prostrating.'"
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا الْفُضَيْلُ، عَنِ الأَعْمَشِ، عَنْ عُمَارَةَ بْنِ عُمَيْرٍ، عَنْ أَبِي مَعْمَرٍ، عَنْ أَبِي مَسْعُودٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ تُجْزِئُ صَلاَةٌ لاَ يُقِيمُ الرَّجُلُ فِيهَا صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ " .
also referenced under: Hadith 1027 (Reference) & Book 11, Hadith 152 (In-Book)