The Messenger of Allah ﷺ said, "A prayer is not valid in which a man does not straighten his back during the bowing and prostration."
حدثنا أحمد بن منيع حدثنا أبو معاوية عن الأعمش عن عمارة بن عمير عن أبي معمر
قال رسول الله ﷺ لا تجزئ صلاة لا يقيم فيها الرجل يعني صلبه في الركوع والسجود
[Sahih]. Also narrated by Ibn Majah (no. 870), Nasai (2/214, no. 1112), and Abu Dawud (no. 855) from the hadith of Amash through this same route. Hafiz graded it hasan in Fath al-Bari, and Ibn Khuzayma graded it sahih (nos. 591, 592, 666), as did Ibn Hibban (nos. 501, 502) and others. Amash explicitly stated that he heard it directly, as recorded by Ibn Hibban.On this topic there are also narrations from Ali bin Shayban [Ibn Majah, no. 871], Anas [Bukhari, no. 742, and Muslim, no. 425], Abu Hurairah [Bukhari, no. 757, and Muslim, no. 397], and Rifa Zurqi [forthcoming, no. 302].