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: "The prayer of a man who does not level his back in bowing and prostrations is insufficient."
This chain is sahih (authentic).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2571