The Messenger of Allah ﷺ said, "No prayer is valid in which a man does not straighten his back during the bowing and prostration."
حدثنا علي بن محمد وعمرو بن عبد الله قالا حدثنا وكيع عن الأعمش عن عمارة عن أبي معمر
قال رسول الله ﷺ لا تجزئ صلاة لا يقيم الرجل فيها صلبه في الركوع والسجود
[Sahih] Abu Dawud narrated it, Prayer, chapter on the prayer of one who does not straighten his back in ruku and sujud, no. 855, from the hadith of Amash with this same chain, and Hafiz graded it hasan in Fath al-Bari.