Hazrat Ali bin Shiban, who was a member of the (delegation that came to serve the Prophet), says that I offered prayer behind the Messenger of Allah (peace be upon him). The Prophet (peace be upon him) glanced sideways and saw a man who was not straightening his back during bowing (ruku) and prostration (sajdah). Then, when the Prophet (peace be upon him) completed the prayer, he said: "O assembly of Muslims, indeed the prayer of the one who does not straighten his back during bowing and prostration is not valid." This is the hadith of Ahmad bin Al-Muqaddam.