"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.'"  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ تُجْزِئُ صَلاَةٌ لاَ يُقِيمُ الرَّجُلُ فِيهَا صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ