(81) باب مَا جَاءَ فِيمَنْ لاَ يُقِيمُ صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ
Abu Mas'ud Al-Ansari [Al-Badri] narrated that :
Allah's Messenger said: "The Salat is not acceptable if a man is not at rest - meaning his back - while bowing and prostrating." (ES)
Reference (ES): Jami` at-Tirmidhi 265 (ES)
In-book reference (ES): Book 2, Hadith 117 (ES)
English translation (ES): Vol. 1, Book 2, Hadith 265 (ES)