It wasnarrated that Abu Mas'ud said:
"The Messenger of Allah (ﷺ)said: 'No prayer is acceptable in which a man does not settle hisspine when bowing and when prostrating.'" (FR)
English reference (FR): Vol. 1, Book 5, Hadith 870 (FR)
Arabic reference (FR): Book 5, Hadith 919 (FR)