This article is part of a free translation of al-Iftqar ila Allah, Lubb al-Ubudiyyah by Ahmad al-Suwayan
Possessing fear of Allah is one of the most sublime qualities of those having iman; Allah says: The believers are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely. Allah also says: And give good tidings to the humble [before their Lord]. Who, when Allah is mentioned, their hearts are fearful. Fearing Allah in open and secret is one of the greatest signs demonstrating need and total deprivation before Him; whoever realises His greatness and omnipotence; His unconquerable authority; His eye that does not sleep; and reveres Him a manner befitting will truly fear Him: But for he who has feared the position of his Lord are two gardens. Allah also says: But as for he who feared the position of his Lord and prevented the soul from [unlawful] inclination, then indeed, Paradise will be [his] refuge. And He says: That is for he who fears My position and fears My threat.
One whose condition is such will have possessed a vigilant heart that shakes out of trepidation and is always in intimately conversant with its Lord. The heart will seek refuge in Him and plead for recourse bring a heart that is poverty-stricken and servile; Allah says: Is one who is devoutly obedient during periods of the night, prostrating and standing [in prayer], fearing the Hereafter and hoping for the mercy of his Lord, [like one who does not]? Say, Are those who know equal to those who do not know? Allah also says: Their sides part [i.e., they arise] from [their] beds; they supplicate their Lord in fear and aspiration  He also says: And those who spend [part of] the night to their Lord prostrating and standing [in prayer] Al-Hasan al-Basri said, Tears run down their cheeks in fear of their Lord.
Contemplate the saying of Allah: Say, Believe in it or do not believe. Indeed, those who were given knowledge before it when it is recited to them, they fall upon their faces in prostration, And they say, Exalted is our Lord! Indeed, the promise of our Lord has been fulfilled. And they fall upon their faces weeping, and it [i.e., the Quran] increases them in humble submission. This is a total need for Allah and dejection displayed before Him; Sayyid Qutub said: They are unable to control themselves; they do not merely prostrate but they fall upon their faces in prostration, then their tongues actuate with the sensation of Allahs greatness and true promise that has become fused into their emotions, Exalted is our Lord! Indeed, the promise of our Lord has been fulfilled. Being affected in this way overwhelms them and words no longer suffice as a manifestation of what is raging in their bosoms tears stream down their faces conveying that profound effect words cannot express.
The condition for genuine fear is that which occurs in private as the heart is attached only to Allah neither turning to anyone nor anything else; Allah says: Indeed, those who fear their Lord in private will have forgiveness and great reward. Allah also says: Who fear their Lord in private, while they are of the Hour apprehensive. And Allah says: And Paradise will be brought near to the righteous, not far, It will be said], This is what you were promised for every returner [to Allah] and keeper [of His covenant]. Who feared the Most Merciful in private and came with a heart returning [in repentance]. The Prophet said in an authentic hadith: Allah will give shade, to seven, on the Day when there will be no shade but His One of the seven mentioned is a person who remembers Allah in seclusion and his eyes are then flooded with tears. Al-Hafiz Ibn Hajar commented concerning on this seclusion, because this will as far as possible be away from ostentation; and the intent here is to be secluded from turning to other than Allah even if one is (physically) in the company of others.
Fear of Allah is a form of worship connected to the heart that drives one to strive, to be earnest, and to have a disposition to perform acts of devotion; the Messenger of Allah said: Whoever fears, will travel during the night, and whoever travels during the night, will reach the destination. For this reason Al-Hafiz Ubaydullah Ibn Jafar said, A slave cannot seek aid for his religion with anything better than the fear of Allah. The reality of this worship of the heart is borne across the limbs and thus we find in the aforementioned hadith about the seven who will have Allahs shade: a man who refuses the call of a charming woman of noble birth for illicit intercourse with her and says: I am afraid of Allah Disobedience presented itself before him in its most accomplished decorative appearance and most splendid manner of temptation and here was a man like any other man yet nothing but the fear of Allah prevented him from her. There is a similar incident to this in the tale of the three persons who were caught stuck in a cave, when one of them supplicated: O Allah! You know that I was in love with a cousin of mine, like the deepest love a man may have for a woman, and she told me that I would not get my desire fulfilled unless I paid her one-hundred Dinars (gold pieces). So, I struggled for it till I gathered the desired amount, and when I sat in between her legs, she told me to be afraid of Allah, and asked me not to deflower her except rightfully (by marriage). So, I got up and left her. O Allah! If You regard that I did it for Your sake, then deliver us from this predicament. And in a variant narration: O Allah! If You consider that I did that for fear of you than please remove the rock. This meek woman resigned herself to him and could only muster the matter of reminding him to fear Allah but his heart awoke and became replete with fear of Allah, which prevented him from committing the sin; the saying of Said Ibn Jubayr is an exceptional definition of fear: (Real) fear means to fear Allah to the extent that your fear prevents your disobedience that is fear.
 Surah al-Anfal, 8:2
 Surah al-Hajj, 22:34-35
 Surah al-Rahman, 55:46
 Surah al-Naziat, 79:40-41
 Surah Ibrahim, 14:14
 Surah al-Zumar, 39:9
 Surah al-Sajdah, 32:16
 Surah al-Furqan, 25:64
 Surah al-Isra, 17:107-109
 Fi Zilal al-Quran, vol. 5 p. 2245
 Surah al-Mulk, 67:12
 Surah al-Anbiya, 21:49
 Surah Qaf, 50:31-33
 Sahih al-Bukhari
 Fath al-Bari, vol. 2 p. 147
 Al-Tirmidhi 4/633, no. 2450
 Siyar al-Alam al-Nubula, vol. 6 p. 9
 Sahih al-Bukhari
 Hilyah al-Awliya, vol. 4 p. 276 and Siyar al-Alam al-Nubula, vol. 6 p. 9
Taken from Islam21c.com: http://www.islam21c.com/spirituality/230-fearing-allah-in-open-and-secret/