" 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: 'The prayer is not valid if a man does not bring his backbone to rest while bowing and prostrating.'"