Sayyiduna Talq bin Ali (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Allah Almighty does not look (with mercy) at the prayer of a servant who does not keep his back straight in his bowing and his prostration."
Hadith Referenceالمعجم الكبير للطبراني / باب الطاء / 8230