Sayyiduna Ali (may Allah be pleased with him) says that when the Messenger of Allah (peace and blessings be upon him) performed ruku‘, he would be so straight that if a bowl of water were placed on his back, it would not spill.
Hadith Referenceالفتح الربانی / أبواب صفة الصلاة / 1687
Hadith Gradingمحدثین:صحیح
Hadith Takhrij«حسن انظر سلسلة الاحاديث الصحيحة: 3331۔ ترقیم بيت الأفكار الدولية: 997»
Brief Explanation
Benefits: … The meaning of this hadith is also established by the narration of Sahih Muslim, whose words are as follows: When the Messenger of Allah (sallallahu alayhi wa sallam) performed bowing (ruku‘), he would neither lower his head nor raise it upwards, rather he would keep it between these two states. Also, the following hadith is noteworthy: Sayyiduna Abu Mas‘ud Badri (radi Allahu anhu) reports that the Messenger of Allah (sallallahu alayhi wa sallam) said: ((The prayer of a man is not sufficient until he straightens his back in bowing (ruku‘) and prostration (sujud).)) … Such a prayer is not sufficient in which a person does not make his back completely straight in bowing and prostration. Acting upon this necessary Sunnah is only possible when a person performs the prayer with composure and tranquility. (Abu Dawud, Tirmidhi, Nasa’i, Ibn Majah)