It is narrated from Hazrat Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: Allah does not even look at the prayer of the person who does not straighten his back between bowing (ruku) and prostration (sajdah). (Who does not perform the pillars with tranquility)
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10799
Hadith Gradingحکم دارالسلام:حدیث حسن،وھذا إسناد مختلف فیہ،عامر: مختلف فیہ