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 اللَّهِ ﷺ «لَا تُجْزِئُ صَلَاةٌ لِأَحَدٍ لَا يُقِيمُ صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ»