It is narrated from Sayyiduna Uqbah bin Amr (Abu Mas'ud) (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The prayer of a man who does not keep his back straight for bowing and prostration is not accepted."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14656