(a) It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Complete the bowing and prostration. By the One in whose hand is my soul, I see you from behind my back when you bow and when you prostrate."
(b) The hadith of Sayyiduna Abu Mas'ud (may Allah be pleased with him) has already passed, that the Prophet (peace and blessings be upon him) said: "The prayer of a man who does not straighten his back in bowing and prostration is incomplete."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2723