It is narrated from Sayyiduna Uqbah bin Amr (Abu Mas'ud) (may Allah be pleased with him) that the Prophet (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."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14652