:sl:
Dear brother,
A good source for the essentials of salat is the book fiqh-us-sunnah by syed sabiq. This book explains the rulings of the madhahib, and is used as a source by almost all of the scholars. It is available for free on the Internet in many places.
The link I use is:
http://www.muhammad.net/ebooks/Fus/index.html
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Volume 1, Page 122a: Obligatory acts of prayer, Ar-Ruku' (Bowing Down)
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 pepole 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.)
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Volume 1, Page 147a: Sunnah acts of prayer, The Manner of Bowing
When one bows, one's hands must reach one's knees. It is sunnah to make the height of the head equal to that of the hips. The hands should be supported by the knees and should be apart from one's sides. The hands should be open upon one's knees and thighs, and the palms should be flat. It is reported that 'Uqbah ibn 'Amr would bow with his arms separated, his hands on his knees, and his fingers opened beyond his knees. He said, "This is how I saw the Messenger of Allah pray." (Related by Ahmad, Abu Dawud and an-Nasa'i.)
Abu Humaid reported that when the Prophet, upon whom be peace, bowed, he would be straight, his head neither up nor down (with respect to his hips), and he would place his hands on his knees as if he was holding them." (Related by an-Nasa'i.)
Muslim records 'Aishah reporting that when the Prophet bowed, his head would be neither risen nor lowered, but rather between those two positions. Said 'Ali, "If you put a cup of water on the back of the Prophet, upon whom be peace, while he was bowing, its contents would not spill." This is related by Ahmad. Abu Dawud recorded it in his Kitab al-Muraseel.
Said Mus'ab ibn Sa'd, "I prayed next to my father. I joined both of my hands and put them between my thighs (while bowing). He stopped me and said, 'We used to do that, but were later ordered (by the Prophet) to put our hands on our knees."' (Related by "the group.")
Volume 1, Page 148: Sunnah acts of prayer, The Remembrance of Allah During the Bowing
It is preferred to remember Allah with the following words, "Subhana Rabiyy al-'Azheem (Glory to my Lord, the Great.)" Reported 'Uqbah ibn 'Amr, "When 'Glorify the name of your Lord, the Great,' was revealed, the Prophet told us, 'Do so in your bowings." This is related by Ahmad, Abu Dawud and others with a good chain.
Reported Huzhaifah, "I prayed with the Messenger of Allah, upon whom be peace, and while bowing he would say, 'Subhana Rabiyy al-'Azheem." (Related by Muslim, Abu Dawud, an-Nasa'i, at-Tirmizhi and Ibn Majah.)
The phrase Subhana Rabiyy al-'Azheem wa bihamdihi has been related through a number of chains, but all of them are weak. Ash-Shaukani maintains, "The different chains support each other. It is perfectly acceptable for one who is praying to limit himself to Subhana Rabiyy al-'Azheem or to add one of the following:
-1- 'Ali reported that while bowing, the Messenger of Allah, upon whom be peace, would say, "O Allah, for You have I bowed, and it is You that I have believed in and to You have I submitted. You are my Lord. My hearing, sight, marrow, bones and nerves and what is carried by my feet are for Allah, the Lord of the Worlds." (Related by Ahmad, Muslim, Abu Dawud and others.)
-2- 'Aishah reported that while bowing and prostrating, the Messenger of Allah, upon whom be peace, would say, "Glorified and Holy are You, Lord of the angels and the souls."
-3- Reported 'Auf ibn Malik, "I prayed with the Messenger of Allah one night. He recited al-Baqarah and while bowing said, 'Glory be to the One of Omnipotence, the Master of the dominions, of grandeur and of honor."' (Related by Abu Dawud, at-Tirmizhi and an-Nasa'i .)
-4- 'Aishah said that when the Prophet, upon whom be peace, bowed or prostrated, he would often say, "Glory and praise be to You, O Allah, our Lord. O Allah, forgive me." This was how he applied the Qur'an. (Related by Ahmad, al-Bukhari, Muslim and others.)