The Book of The At-Tatbiq (Clasping One's Hands Together)
12/54. Chapter:
Maintaing one's back (at ease) when prostrating
Sunan an-Nasai
أَخْبَرَنَا عَلِيُّ بْنُ خَشْرَمٍ الْمَرْوَزِيُّ، قَالَ أَنْبَأَنَا عِيسَى، - وَهُوَ ابْنُ يُونُسَ - عَنِ الأَعْمَشِ، عَنْ عُمَارَةَ، 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 " .
It was narrated that Abu Mas'ud said: "The Messenger of Allah (ﷺ) said: 'No prayer is valid in which a man does not maintain his back (at ease) when bowing and prostrating.'"
Reference : Sunan an-Nasa'i 1111In-book reference : Book 12, Hadith 83English translation : Vol. 2, Book 12, Hadith 1112