صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب الأذان والإقامةChapters on Adhan and Iqamah
(149) بَابُ ذِكْرِ الْبَيَانِ أَنَّ صَلَاةَ مَنْ لَا يُقِيمُ صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ غَيْرُ مُجْزِئَةٍ،Chapter 149: Clarification that prayer is invalid if one does not straighten the back in bowing and prostration
Hadith Q591
لَا أَنَّهَا نَاقِصَةٌ مُجْزِئَةٌ كَمَا تَوَهَّمَ بَعْضُ مَنْ يَدَّعِي الْعِلْمَ.
It is not that the prayer becomes deficient, but it suffices, just as some people who claim knowledge think.
