Sunan Ibn Majah 871

It was narrated that ‘Ali bin Shaiban who was part of a delegation (to the Prophet (ﷺ)) said:“We set out until we came to the Messenger of Allah (ﷺ), and we gave him our oath of allegiance and performed prayer behind him. He glanced out of the corner of his eye at a man who was not settling his spine when he bowed and prostrated. When the Prophet (ﷺ) finished the prayer, he said: ‘O Muslims, there is no prayer for the one who does not settle his spine when bowing and prostrating.’”