ENOfficial
Abu Mas'ud al-Ansari reported God’s Messenger as saying, “A man’s prayer does not avail him unless he keeps his back steady when bowing and prostrating himself.” Abu Dawud, Tirmidhi, Nasa’i, Ibn Majah and Darimi transmitted it, and Tirmidhi said this is a hasan sahih tradition.
Source: sunnah.com
Other translations
Arabic
عَنْ أَبِي مَسْعُودِ الْأَنْصَارِيِّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا تُجْزِئُ صَلَاةُ الرَّجُلِ حَتَّى يُقِيمَ ظَهْرَهُ فِي الرُّكُوعِ وَالسُّجُودِ» . رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَالنَّسَائِيُّ وَابْنُ مَاجَهْ وَالدَّارِمِيُّ وَقَالَ التِّرْمِذِيُّ: هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ
sunnah.com