It wasnarrated that 'Ali bin Shaiban who was part of a delegation(to theProphet (ﷺ)) said:
"We set out until we came to theMessenger ofAllah (ﷺ), and we gave him our oath of allegiance andperformedprayer behind him. He glanced out of the corner of his eyeat a manwho was not settling his spine when he bowed and prostrated.When theProphet (ﷺ) finished the prayer, he said: 'O Muslims, thereis noprayer for the one who does not settle his spine when bowing andprostrating.'" (ID)
English reference (ID): Vol. 1, Book 5, Hadith 871 (ID)
Arabic reference (ID): Book 5, Hadith 920 (ID)