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: "A prayer in which a man does not keep his back straight in bowing (ruku') and prostration (sujud) is not sufficient."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14650