(88) باب إِقَامَةِ الصُّلْبِ فِي الرُّكُوعِ
It was narrated that Abu Mas'ud said:
"The Messenger of Allah (ﷺ) said: 'The prayer is not valid if a man does not bring his backbone to rest while bowing and prostrating.'"
Reference: Sunan an-Nasa'i 1027
In-book reference: Book 11, Hadith 152
English translation: Vol. 2, Book 11, Hadith 1028