Abdur Rahman bin Ali bin Shaiban narrates from his father, who was one of the six members of a delegation. He states: We presented ourselves before the Prophet Muhammad (peace be upon him). We performed prayer behind him. The Prophet Muhammad (peace be upon him) saw, from the corner of his eyes, a person who was not keeping his back straight during bowing (ruku) and prostration (sajda). So the Prophet Muhammad (peace be upon him) said: The prayer of that person is not valid who does not keep his back straight.