It is stated in Misbahush-Shari`ah that as-Sadiq (AS) said: "No servant bows in the ruku `for Allah, in a true way, unless Allah adorns him with the light of His Brilliance, and shades him with the shade of His Majesty, and clothes him with the gown of His chosen friends. The ruku ` is first and the sujud is second. The one who performed the first is fit for the second. The ruku ` is politeness, and the sujud is proximity. So, the one who is not well-mannered, is unfit for proximity. Therefore, perform the ruku ` like the one who is submitting to Allah with his heart, humble and afraid under His Sovereignty, drooping to Him his limbs like the one who is afraid and grieved for what one loses of the benefit of those who perform the ruku`. It is narrated that ar-Rabi` ibn Khuthaym used to keep indulged in a single ruku `all the night till dawn. In the morning he would raise his head and say: `Oh, the sincere ones have passed us and we are left behind.' Complete your ruku ` by levelling your back, and rebuke yourself for being vigilant in serving Him except with His help. Protect your heart against Satan's whispers, deceits and traps. Allah, the Exalted, raises His servants as much as they show humility to Him, and He guides them to the principles of humility and submission as much as His Greatness knows of their secrets." 
This noble hadith contains signs, good tidings, disciplines and instructions, as "adorning" with the "light of Allah's Brilliance", being under the "shade of Allah's Majesty" and wearing the "gown of Allah's chosen ones" are good tidings of attaining to the state of learning the Names: "And He taught Adam all the names".  Of that state is the taking of the state of attributive annihilation and the state of wakefulness, because Allah's adorning the servant with the state of "the light of His Brilliance" means taking him to the state of Names, which is the reality of teaching man. Taking him under the shade and shadow of "Majesty", which is of the Names of Power, and placing him in His Court, refer to annihilating the servant of himself. After this state, clothing him in "the gown of the chosen ones" is survival after annihilation. From this it is realized that the sujud is self-annihilation, as is said by the people of knowledge. The ruku` is first, and it is these states, and the sujud is second, which is nothing but the state of annihilation in the Essence.
It is also realized that the Absolute Proximity, which happens in the sujud, is not accessible except through the true ruku `. Whoever wants to be fit for the second, must acquire the discipline of the ruku ` and its proximity.
After stating the delicacies and secrets of the ruku ` and sujud, he refers to their hearty disciplines to suit the middle class. These are among the general matters, which we have referred to in the Preliminaries; and some of them belong to ruku `. Now, as most of these things have already been stated, we leave out further explanations.
. Misbahush-Shari`ah, p. 12. Biharul Anwar, vol. 89, p. 108.
. Surah al-Baqarah: 31.