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 and prostration is not sufficient," or he said: "when he bows and when he prostrates." Abu Awanah was unsure which words were used.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14653