The Messenger of Allah ﷺ said, "A man's prayer is not valid unless he straightens his back during the bowing and prostration."
قال رسول الله ﷺ: لا تجزئ صلاة الرجل حتى يقيم ظهره في الركوع والسجود.
Abu Dawud (855), Tirmidhi (265), Nasai (1027), and Ibn Majah (870) all narrated it, each of them through the route of Amash, from Umara bin Umayr, from Abu Muamar, from Abu Masud Ansari Badri, who related the same. In Nasai's version, it reads, "until the man straightens his back in ruku and sujud," while Tirmidhi treated the word "his back" as an explanatory addition. Tirmidhi said, hasan sahih. I say, it is as he said, and its chain of transmission is sahih. Ibn Khuzayma (592) and Ibn Hibban (1893) also graded it sahih, both narrating it through this same route.As for Abu Masud, his name was Uqba bin Amr, and there is disagreement about his attribution to Badr. Some say he did not witness Badr, and was only linked to it because he settled by a body of water called Badr, but the correct view is that he was indeed among those who witnessed Badr. This is the position held by Bukhari, Muslim, Abu Ubayd, and Hakim Abu Ahmad. See Fath al-Bari (7/246).