It is narrated from Sayyiduna Abu Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The prayer of anyone (or he said: of a man) in which he does not keep his back straight in bowing and prostration is not sufficient."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14651