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: "There is no hope for a prayer in which a man does not keep his back straight in bowing and prostration."
Sufyan said: Al-A'mash said it like this, "there is no hope", meaning "it is not sufficient".
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14654