The Messenger of Allah ﷺ said, "A man's prayer is not valid until he straightens his back in bowing and prostration."
حدثنا حفص بن عمر النمري حدثنا شعبة عن سليمان عن عمارة بن عمير عن أبي معمر
قال رسول الله ﷺ لا تجزئ صلاة الرجل حتى يقيم ظهره في الركوع والسجود
[Its chain is sahih]: Tirmidhi transmitted it, Prayer, chapter on those who do not straighten their backs in ruku and sujud, no. 265, from the hadith of Sulayman Amash, with this wording, and he said, hasan sahih. Ibn Majah also narrated it, no. 870.