“The Messenger of Allah ﷺ said: ‘No prayer is acceptable in which a man does not settle his spine when bowing and when prostrating.’” (Using translation from Ibn Mājah 870)  

قَالَ رَسُولُ اللهِ ﷺ «لَا تُجْزِئُ صَلَاةٌ لَا يُقِيمُ الرَّجُلُ فِيهَا صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ»