The Messenger of Allah ﷺ said, "Allah does not look at the prayer of a man who does not straighten his back between his bowing and his prostration."
حدثنا يحيى بن آدم حدثنا عامر بن يساف حدثنا يحيى بن أبي كثير عن عبد الله بن بدر الحنفي
قال رسول الله ﷺ لا ينظر الله إلى صلاة رجل لا يقيم صلبه بين ركوعه وسجوده
Its isnad is hasan. As for Abdullah bin Badr Hanafi, he is trustworthy, except that in Haythami's copy the name appears as Abdullah bin Zayd Hanafi, so he said, "I do not know him," 2/120, but he did not refer to the discussion regarding Amir bin Yasaf, for many have declared him daif and many have declared him trustworthy. I only graded it hasan because of its corroborating narration and supporting evidence. The hadith was narrated by Abu Dawud 1/226 no. 855, Tirmidhi 2/51 no. 265, and Nasai 2/183.