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