Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.122A.

There is a consensus on the obligatory nature of the ruku’. Says Allah, “O you who believe, bow down and prostrate yourselves…” The position of ruku’ is established by bending over, putting one’s hands on one’s knees, and remaining in that position until he attains “calmness.” In another hadith the Prophet said, “Then bow until you attain calmness while your are bowing.” Abu Qatadah related that the Prophet, upon whom be peace, said, “The worst people are the thieves who steal part of the prayer.” He was asked how this was done, and he replied, “He does not complete his bowings and prostrations,” or he said, “He does not straighten his back during his bowings and prostrations.”

As to its authenticity, the report is related by Ahmad, at-Tabarani, Ibn Khuzaimah and al-Hakim, who consider its chain as sahih.

Abu Mas’ud al-Badri reported that the Prophet, upon whom be peace, said, “The prayer of one who does not straighten his back in his bendings and prostrations is not accomplished.” This hadith is related by “the five,” and Ibn Khuzaimah, Ibn Hibban, at-Tabarani and al-Baihaqi, who consider its chain as sahih, while at-Tirmizhi grades it as hassan sahih.

Knowledgeable companions act according to the principle that a person is to make his back straight during his bowings and prostrations. Huzhaifah saw someone who did not straighten his back during his bowings and prostrations, and told him, “You have not prayed. And if you were to die, you would not die in the way of Allah and His Messenger.” (Related by al-Bukhari.)