It is narrated from Ali bin Shayban (may Allah be pleased with him) that he was in the delegation of Banu Suhaym that came to the Prophet (peace and blessings be upon him). We prayed a prayer with the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) glanced from the corner of his eye at a man who was not straightening his back in bowing and prostration. When he (peace and blessings be upon him) said the salam, he said: "O people! There is no prayer for a man who does not straighten his back in bowing and prostration."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5213