It is narrated from Sayyiduna Taliq bin Ali that the Prophet (peace and blessings be upon him) said: Allah does not look at the prayer of the person who does not straighten his back between bowing (ruku) and prostration (sajdah).
Hadith Referenceمسند احمد / مسند المدنيين رضي الله عنهم اجمعين / 16284
Hadith Gradingحکم دارالسلام:إسناده ضعيف لضعف أيوب بن عتبة