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 comer 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. (Sahih)