The Prophet ﷺ said: A man's prayer does not avail him unless he keeps his back steady when bowing and prostrating.  

قَالَ رَسُولُ اللَّهِ ﷺ لاَ تُجْزِئُ صَلاَةُ الرَّجُلِ حَتَّى يُقِيمَ ظَهْرَهُ فِي الرُّكُوعِ وَالسُّجُودِ  

The Prophet ﷺ said: A man's prayer does not avail him unless he keeps his back steady when bowing and prostrating. (Using translation from Abū Dāʾūd 855)   

قَالَ رَسُولُ اللهِ ﷺ لَا تُجْزِئُ صَلَاةٌ لِأَحَدٍ لَا يُقِيمُ فِيهَا ظَهْرَهُ فِي الرُّكُوعِ وَالسُّجُودِ  

[Machine] The Prophet ﷺ said, "The prayer of a man or anyone who does not straighten his back during bowing and prostration is not accepted."  

عَنِ النَّبِيِّ ﷺ قَالَ لَا تُجْزِئُ صَلَاة الرجل أَوْ أَحَدٍ لَا يُقِيمُ ظَهْرَهُ فِي الرُّكُوعِ وَالسُّجُودِ  

[Machine] The Messenger of Allah ﷺ said, "The prayer of someone is not valid or it was said, 'The person does not properly straighten their back during the bowing and prostration.'"  

قَالَ رَسُولُ اللهِ ﷺ لَا تُجْزِئُ صَلَاةُ أَحَدٍ أَوْ قَالَ الرَّجُلِ لَا يُقِيمُ ظَهْرَهُ فِي الرُّكُوعِ وَالسُّجُودِ  

“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)   

النَّبِيَّ ﷺ قَالَ لَا تُجْزِي صَلَاةٌ لَا يُقِيمُ الرَّجُلُ فِيهَا صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ  

"لا تُجْزِيءُ صَلاةٌ لَا يُقِيمُ الرَّجلُ فيهَا صُلبَهُ في الرُّكُوع وَالسُّجُودِ".