"Verily they used to hasten on to do good deeds, and they used to call on Us with hope and fear, and used to humble themselves before Us."
[Soorah al-Anbiyaa' 21:90] and He said: 
   
"And for the men and women who humble (before their Lord - Allaah)... Allaah has prepared for them forgiveness and a great reward."
[Soorah al-Ahzaab 33:35]   

The most honourable worshippers who preserve it are those of the believers having been described with al-Khushoo', as He says:
 
"Successful indeed are the believers. Those who offer their prayer with al-Khushoo."
[Soorah al-Muminoon 23:1-2]

And those endued with knowledge have been described with al-Khushoo' and truthful speech, as He says: 

"Verily! Those who were given knowledge before it, when it was recited to them, fall down on their faces in humble prostration. And they say: Glory be to our Lord! Truly, the promise of our Lord must be fulfilled. And they fall down on their faces weeping and it adds to their humility." [Soorah al-Israa' 17:107-109]    
  
The foundation of al-Khushoo' is: Soft-heartedness, tranquil, humble, subservient and yearning. So when the heart has Khushoo' it is followed by the Khushoo' of all the limbs and bones, since they all follow it ( the heart), as he, sallAllaahu 'alayhi wa sallam said: 
  
"Indeed in the body there is a piece of flesh, if it is sound then the whole body is sound; and if it be corrupt then the whole body is corrupt. Indeed it is the heart." 

So when the heart has Khushoo' then the hearing, the sight, the head, the face and all of the bones, even to the extent of the speech, will have Khushoo'. Such that the Prophet sallAllaahu 'alayhi wa sallam would say in the rukoo' of the Prayer:
 
   "Humbled to You are my hearing, my seeing, my marrow, my bone."
 
 In another narration:

   "And whatever my feet carry."
 
 When some of the Salaf saw a man moving his hand during Prayer, they would say: "If the heart had Khushoo' the limbs would have Khushoo'." This is narrated from Hudhayfah radiAllaahu 'anhu and from Sa'eed ibn al-Musayyib. It is related in marfoo' form. However, its chain of transmission (isnaad) is not authentic. Al-Mas'oodee relates from Abu Sinaan from a narrator from 'Alee ibn Abee Taalib radiAllaahu 'anhu, that he said about His - Most High saying:

 "Those who offer their prayers with all Khushoo'."
[soorah al-Muminoon 23:2]   
 
  He said: 
  
"It is the heart that has al-Khushoo' and is gentle towards the other muslim and he does not look around in his Prayer."

'Ataa ibn as-Saa'ib related from a man from 'Alee radiAllaahu 'anhu: 'al-Khushoo' is the khushoo' of the heart, whereby it does not look to the right nor the left." 'Alee ibn Abee Talhaa related from Ibn 'Abbaas radiAllaahu 'anhumaa that he said about the saying of the Most High:  Who have Khushoo' in their Prayer. He said: They are fearful and tranquil."

Ibn Shawdhab related from al-Hasan rahimahullaahu ta'aalaa, that he said: "They had al-Khushoo' in their hearts, so they could concentrate their sight in the Prayer." Ibn Abee Najeeh related from Mujaahid rahimahullaahu ta'aala, about His saying: 

 "Used to humble themselves before Us."
[soorah Anbiyaa 21:90]

  That he said: "Those who have humility (al-Mutawaadi'een). "
Allaah - Most High - in His Book, has characterised the Earth with al-Khushoo', as He says: 
  
"And among His Signs is this: You see the earth barren, but when We send down rain to it, it is stirred to life and yields increase." [soorah Haa Meem 41:39]*   
 

 So it is stirred to life and yields increase shows that the original state of the earth is one of al-Khushoo' and as such it remains stationary. Likewise is the heart. When it is in the state of al-Khushoo' the worries and anxieties leave and it becomes tranquil; and the following of desires will be subdued and humbled before Allaah. This results in lifting and obliterating pride; and when this tranquility is in the heart, then the bones, limbs and all movements become humbled - even to the extent of the voice. Allaah - Most High - describes the voices with al-Khushoo' in His saying:

 "And all voices will be humbled for the Most Beneficent (Allaah)"
[soorah Taa Haa 20:108]    

So al-Khushoo' of the voice is its silence and lowering, after its being raised. Similarly, the faces and the sight of the disbelievers on the Day of Judgement, has been described with al-Khushoo' and this is a proof that their whole body will be in a state of al-Khushoo' (due to fear of Allaah). On the other hand, when a person is forced to produce al-Khushoo' in his body and limbs - whilst the heart is devoid of it - then the person is described as having the Khushoo' of hypocrisy (al-Khushoo un-Nifaaq). It is this that some of the Salaf sought refuge from, as one of them said: I seek refuge with Allaah from the Khushoo' of Hypocrisy. It was said: What is the Khushoo' of Hypocrisy? He said: That the body is seen to be in the state of al-Khushoo', but the heart - it is not in the state of al-Khushoo'.  'Umar radiAllaahu 'anhu saw a young man lowering his head, so he said to him:

"What is this! Raise your head, for indeed this (kind of) Khushoo' does not increase what is in the heart! Whosoever displays al-Khushoo' that does not stem from the heart, then it is Hypocrisy added to Hypocrisy."

  The basis of al-kushoo' which exists in the heart is brought about when one recognises Allaah, realizing His greatness, His power and His Perfection; therefore whoever recognises Allaah more, then He has the greater al-kushoo' towards Him Azza wa Jaal. The differences of al-kushoo' that exist are due to the variation of having knowledge of the greatness of Allaah Azza wa Jaal and witnessing of the attributes of Allaah Azza wa Jaal which bring about this al-kushoo'. As such when a person is in the state of al-kushoo', the heart is strong in remembering and 'witnessing' Allaah Azza wa Jaal ; where Allaah is 'near' {in knowledge} to His slave, the heart is constantly communicating in secret to Allaah Azza wa Jaal and the heart is in humility where the person is ever conscious of his movement. 

 Whoever is in al-kushoo' contemplating about the Perfection and Greatness of Allaah Azza wa Jaal will be immersed in the sea of Love and he will be ever yearning to meet and to look at the Face of Allaah. And whoever is in al-kushoo' meditating about the terror of the punishment of Allaah will feel remorse and extremely fearful of Allaah. This is how Allaah makes the hearts of His creatures bow toward Him. Where He is near to those of His slaves who 'speak' to him through the Salaah, and those who prostrate to Him, as He draws Himself closer to those of His slaves who implore humbly for assistance or seeking forgiveness in the early hours of the dawn, and He responds and answers the supplications of His slaves and He does force anyone of His creatures to bow and prostrate to Him with the way that will be closest to Him though He is the One who answers every supplication.  In the Kitabus-Az-Zuhd, Imam Ahmad narrated from Imraan al-Qasirr {or al-Juni [Malay translators addition] } that Musa asked Allaah : "O Rabb, where can I achieve your pleasure (ridha)?" and Allaah replied : "Seek My pleasure when the heart of my slave is in humility due to fearing Me. Verily I draw closer to them one handspan everyday and if this does not happen then they are destroyed."