The Prophet ﷺ said, "The prayer of a man, or of anyone, is not valid if he does not straighten his back during bowing and prostration."
حدثنا حسين بن محمد حدثنا شعبة عن سليمان قال سمعت عمارة بن عمير التيمي يحدث عن أبي معمر الأزدي
عن النبي ﷺ قال لا تجزئ صلاة لرجل أو لأحد لا يقيم ظهره في الركوع والسجود
Its isnad is sahih, its narrators are famous and trustworthy. Umara bin Umayr Kufi is a trustworthy and firmly reliable narrator, previously mentioned. Abu Muamar Azdi is Abdullah bin Sukhayra, the well-known trustworthy narrator. The hadith has already appeared at 16249.