Allaah, Most High, says,
"Has not the time come for the hearts of those who believe (in the Oneness of Allah - Islamic Monotheism) to be affected by Allah's Reminder (this Quran), and that which has been revealed of the truth, lest they become as those who received the Scripture [the Taurat (Torah) and the Injeel (Gospel)] before (i.e. Jews and Christians), and the term was prolonged for them and so their hearts were hardened? And many of them were Fasiqun (rebellious, disobedient to Allah)." (Soorah al-Hadeed, 57:16)
Sayyiduna 'Ibn Mas'ood رضي الله عنه said, "The time between our accepting Islaam and being rebuked by this verse was four years." 
Sayyiduna Ibn 'Abbaas رضي الله عنهما said, "Allaah سبحانه و تعالى granted time and leeway to the hearts of the believers, then, at the turn of the thirteenth year after the revelation had started, He rebuked them." 
Allaah, Most High, says,
"Successful indeed are the believers. Those who offer their prayers with all solemnity and full submissiveness." (Soorah al-Mu'minoon, 23: 1-2)
Linguistically, Khushoo' means sinking, subservience, and stillness. Allaah, Most High, says,
"On that Day mankind will follow strictly (the voice of) Allah's caller, no crookedness (that is without going to the right or left of that voice) will they show him (Allah's caller). And all voices will be humbled for the Most Beneficent (Allah), and nothing shall you hear but the low voice of their footsteps." (Soorah Taa Haa, 20:108)
i.e. still and humbled. In this respect, the earth has been described as having Khushoo', i.e. its being dry, bare, low and not being elevated with plant and vegetation. Allaah, Most High, says,
"And among His Signs (in this), that you see the earth barren, but when We send down water (rain) to it, it is stirred to life and growth (of vegetations). Verily, He Who gives it life, surely, (He) is Able to give life to the dead (on the Day of Resurrection). Indeed! He is Able to do all things." (Soorah al-Fussilat, 41:39)
(Technically), Khushoo' refers to the heart standing before the Lord in submissiveness, subservience, and focusing on Him.
It is also said that Khushoo' is to submit to the truth, however (the truth is that) this is one of its results. Therefore, amongst the signs of Khushoo' is that when the servant opposes the truth and is reminded of it, he accepts and willingly returns to it.
It is said that Khushoo' is the abating of the flames of lusts in the servants' breast and the dissipation of their smoke; replacing in their stead the blaze of the greatness (of Allaah سبحانه و تعالى) in the heart.
Junaid رحمه الله said, "Khushoo' is the humbling of hearts to the One who knows the unseen."
The Gnostics have agreed that the seat of Khushoo' is the heart and that its fruits sprout on the limbs and they display it. The Prophet صلى الله عليه وسلم saw a man playing with his beard while praying and remarked, "If the heart of this person was humble, so too would his limbs be." 
The Prophet صلى الله عليه وسلم said, "Taqwaa  is here," pointing to his breast, and he said this three times. 
One of the Gnostics said, "Fine conduct on the outer is a clear indication of the inward." One of them saw a person showing Khushoo' on his shoulders and body and said, "O such-and-such, Khushoo' he here," pointing to his breast, "not here," pointing to his shoulder.
One of the Companions رضي الله عنهم, Sayyiduna Hudhayfah رضي الله عنه, would say, "Take refuge with Allaah from hypocritical Khushoo'." When asked what it was, he replied, "That you see the body humble and submissive while the heart is not." 
Sayyiduna 'Umar رضي الله عنه saw a person with his neck bowed in prayer and said, "O such-and-such, raise your head, for Khushoo' lies not on the neck but in the heart."
[Ummul Mu'mineen] 'Aa'isha [as-Siddeeqah bint as-Siddeeq] رضي الله عنهما saw some youth trying to walk with an air of quietude, so she asked, "Who are they?" Her colleagues replied, "Ascetics." She said, "When 'Umar Ibn al-Khattaab رضي الله عنه walked, he walked rapidly; when he spoke, he was heard; when he struck someone, it hurt; when he fed (the poor), he made sure they ate to their fill; he was the true ascetic!"
Fudayl Ibn 'Iyaad رحمه الله said, "It used to be disliked for a person to display more Khushoo' than was in his heart."
Sayyiduna Hudhayfah رضي الله عنه said, "The first thing you will lose of your religion will be Khushoo' and the last thing you will lose of your religion will be the prayer, and it is well possible that there is no good in a person who prays, and soon will come a time when you shall enter a large Masjid and not see a single person with Khushoo'."
Sahl رحمه الله said, "The person whose heart has Khushoo' will not go near Shaytaan."  
 Saheeh Muslim #3027
 as-Suyootee رحمه الله: ad-Durr, vol 14, p277 who references it to Ibn Abee Haatim and Ibn Mardawayh
 It was ruled mawdoo' (fabricated) by Shaykh al-Albaanee رحمه الله, ad-Da'eefah #110 and al-Irwaa' #373
 Taqwaa: to ward off evil.
Talq Ibn Habeeb رحمه الله said, upon being asked about Taqwaa, "That you perform the obedience of Allaah سبحانه و تعالى upon a light from Allaah سبحانه و تعالى, hoping for the reward of Allaah سبحانه و تعالى. You leave disobedience to Allaah سبحانه و تعالى upon a light from Allaah سبحانه و تعالى , fearing the punishment of Allaah سبحانه و تعالى." Recorded by Ibn al-Mubaarak رحمه الله, az-Zuhd #473 with a saheeh isnaad.
Imaam Ibn al-Qayyim al-Jawziyyah رحمه الله, ar-Risaalah at-Tabookiyyah, p27, said, "This is the best that has been said concerning the definition of Taqwaa for indeed every action must have a beginning cause to it and an objective. An action can never be considered to be obedience and a cause to draw one closer to Allaah سبحانه و تعالى until its point of commencement and cause be unadulterated faith, not habit, nor base desires, nor the wish for praise and position, nor other such things. Its objective must be the reward that lies with Allaah سبحانه و تعالى and His good-pleasure, this being the definition of ihtisaab. This is why we often find the combined mention of these two foundations such as in the saying of the Messenger of Allaah صلى الله عليه وسلم, "Whoever fasts the month of Ramadaan out of faith and ihtisaab..."
His saying, "upon a light from Allaah" points to the first foundation which is faith. And his saying, "hoping for the reward from Allaah" points to the second foundation which is ihtisaab."
 Saheeh Muslim #2564 on the authority of Sayyiduna Abu Hurayrah (may Allaah be pleased with him)
 Imaam Ahmad رحمه الله, az-Zuhd, p142 and Ibn al-Mubaarak رحمه الله, az-Zuhd, p143 on the authority of Sayyiduna Abud-Dardaa' (may Allaah be pleased with him)
 Ibn al-Qayyim رحمه الله, Madaarij as-Saalikeen, vol 1, pp 417-418
 Humility in Prayer, pp 66-70