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