Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Allah Almighty does not look at the prayer of a man who does not keep his back straight between his ruku' (bowing) and his sujud (prostration)."
Imam Ahmad recorded this narration as reported by Abdullah bin Zayd al-Hanafi from Sayyiduna Abu Hurayrah (may Allah be pleased with him), and I have not found anyone who has written a biography of this man.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2723
Hadith Gradingمحدثین:إسنادہ ضعیف کیونکہ اس کا ایک راوی عبد اللہ بن زید الحنفی مجہول