The Messenger of Allah ﷺ said, "No prayer is valid in which a man does not straighten his back during the bowing and prostration."
أخبرنا قتيبة قال حدثنا الفضيل عن الأعمش عن عمارة بن عمير عن أبي معمر
قال رسول الله ﷺ لا تجزئ صلاة لا يقيم الرجل فيها صلبه في الركوع والسجود
[Its chain is sahih] Abu Dawud narrated it in the Book of Prayer, chapter on the prayer of one who does not straighten his back in ruku and sujud, no. 855, from the hadith of Sulayman Amash, with the same wording. It is also found in al-Kubra, no. 1100, and Ibn Khuzayma, no. 291, 666, 592, graded it sahih, as did Ibn Hibban, no. 501, 502 (Mawarid), and Tirmidhi, no. 265, said, "Hasan sahih." Amash explicitly stated that he heard it directly, according to him.