Hazrat Abdur Rahman bin Ali bin Shaiban narrates from his respected father, Sayyiduna Ali bin Shaiban (may Allah be pleased with him), that he was a member of the delegation (that came to the service of the Noble Prophet, peace and blessings be upon him). He says: We performed prayer behind the Noble Prophet (peace and blessings be upon him), and he glanced with the corner of his eye at a man who was not straightening his back in bowing (ruku) and prostration (sujud). Then, when the Messenger of Allah (peace and blessings be upon him) completed the prayer, he said: "O assembly of Muslims, indeed the prayer of the one who does not straighten his back in bowing and prostration is not valid." This is the hadith of Ahmad bin al-Muqaddam.