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.'" (ES)
English reference (ES): Vol. 1, Book 5, Hadith 870 (ES)
Arabic reference (ES): Book 5, Hadith 919 (ES)