Possessing Taqwaa i.e. Fear of Allah (Subhaanahu Wa Ta’aalaa), is one of the most sublime qualities of those with Imaan. Allah (Subhaanahu Wa Ta’aalaa) says:

إِنَّمَا الْمُؤْمِنُونَ الَّذِينَ إِذَا ذُكِرَ اللَّهُ وَجِلَتْ قُلُوبُهُمْ وَإِذَا تُلِيَتْ عَلَيْهِمْ آيَاتُهُ زَادَتْهُمْ إِيمَانًا وَعَلَى رَبِّهِمْ يَتَوَكَّلُونَ [الأنفال: 2]

“Certainly, the believers are those whose hearts are filled with awe when (the name of) Allah is mentioned; and when His verses are recited to them, it makes them more developed in faith; and in their Lord they place their trust.” (Al-Anfaal 8:2)

Allah (Subhaanahu Wa Ta’aalaa) also says:

وَبَشِّرِ الْمُخْبِتِينَ[]الَّذِينَ إِذَا ذُكِرَ اللَّهُ وَجِلَتْ قُلُوبُهُمْ [الحج: 34-35]

“…….And give good news to those who turn to Him with humbleness, those whose hearts are filled with awe when Allah is remembered……..” (Al-Hajj 22:34-35)

Fearing Allah (Subhaanahu Wa Ta’aalaa) in open and secret is one of the greatest signs demonstrating need and total deprivation before Him; whoever realizes His greatness and omnipotence; His unconquerable authority; His eye that does not sleep; and reveres Him a befitting manner will truly fear Him.

وَلِمَنْ خَافَ مَقَامَ رَبِّهِ جَنَّتَانِ [الرحمن: 46]

“And for the one who is fearful of having to stand before his Lord, there are two gardens.” (Ar-Rahmaan 55:46)

Allah (Subhaanahu Wa Ta’aalaa) also says:

وَأَمَّا مَنْ خَافَ مَقَامَ رَبِّهِ وَنَهَى النَّفْسَ عَنِ الْهَوَى[] فَإِنَّ الْجَنَّةَ هِيَ الْمَأْوَى [النازعات: 40-41]

“whereas for the one who feared to stand before his Lord, and restrained his self from the (evil) desire, the Paradise will be the abode.” (An-Naazi’aat 79:40-41)

And He says:

ذَلِكَ لِمَنْ خَافَ مَقَامِي وَخَافَ وَعِيدِ [إبراهيم: 14]

“This is for the one who is fearful of standing before Me and is fearful of My warning” (Ibrahim 14:14)

One whose condition is such will have possessed a vigilant heart that shakes out of trepidation and is always intimately conversant with its Lord. The heart will seek refuge in Him and plead for recourse, being a heart that is poverty-stricken and servile. Allah (Subhaanahu Wa Ta’aalaa) says:

أَمَّنْ هُوَ قَانِتٌ آنَاءَ اللَّيْلِ سَاجِدًا وَقَائِمًا يَحْذَرُ الْآخِرَةَ وَيَرْجُو رَحْمَةَ رَبِّهِ قُلْ هَلْ يَسْتَوِي الَّذِينَ يَعْلَمُونَ وَالَّذِينَ لَا يَعْلَمُونَ إِنَّمَا يَتَذَكَّرُ أُولُو الْأَلْبَابِ [الزمر: 9]

“Can (such people be equal to) the one who worships during the hours of night, prostrating himself and standing, fearing the Hereafter and having hopes in his Lord‘s mercy? Say: ‘Can those who know and those who do not know become equal?’ It is only the people of understanding who are receptive of the advice.” (Az-Zumar 39:9)

Allah (Subhaanahu Wa Ta’aalaa) also says:

تَتَجَافَى جُنُوبُهُمْ عَنِ الْمَضَاجِعِ يَدْعُونَ رَبَّهُمْ خَوْفًا وَطَمَعًا [السجدة: 16]

“Their sides remain apart from their beds. They call their Lord with fear and hope ……… ” (Sajdah 32:16)

He also says:

وَالَّذِينَ يَبِيتُونَ لِرَبِّهِمْ سُجَّدًا وَقِيَامًا [الفرقان: 64]

“and those who pass the night prostrating themselves and standing before their Lord.” (Al-Furqaan 25:64)

Al-Ḥasan al-Baṣri said: ‘Tears run down their cheeks in fear of their Lord.’ Contemplate the saying of Allah (Subhaanahu Wa Ta’aalaa): “Say: ‘Believe in it or do not believe in it; indeed, when it is recited to those who were given knowledge before it, they fall down on their faces in prostration, and say: ‘Pure is our Lord. Certainly, the word of our Lord was sure to be fulfilled’. They fall down on their faces weeping, and it increases humbleness in their hearts.” (Al-Israa 17:107-109)

This illustrates a total need for Allah (Subhaanahu Wa Ta’aalaa) and dejection displayed before Him.

The condition for genuine fear is that which occurs in private as the heart is attached only to Allah (Subhaanahu Wa Ta’aalaa) neither turning to anyone nor anything else. Allah (Subhaanahu Wa Ta’aalaa) says:

إِنَّ الَّذِينَ يَخْشَوْنَ رَبَّهُمْ بِالْغَيْبِ لَهُمْ مَغْفِرَةٌ وَأَجْرٌ كَبِيرٌ [الملك: 12]

“Surely, for those who have awe of their Lord without seeing (Him), there is forgiveness and a big reward.”(Al-Mulk 67:12)

Allah (Subhaanahu Wa Ta’aalaa) also says:

الَّذِينَ يَخْشَوْنَ رَبَّهُمْ بِالْغَيْبِ وَهُمْ مِنَ السَّاعَةِ مُشْفِقُونَ [الأنبياء: 49]

“those who have awe of their Lord without having seen Him, and are fearful of the Hour (of Judgment).” (Al-Anbiyaa 21:49)

And Allah (Subhaanahu Wa Ta’aalaa) says:

وَأُزْلِفَتِ الْجَنَّةُ لِلْمُتَّقِينَ غَيْرَ بَعِيدٍ[] هَذَا مَا تُوعَدُونَ لِكُلِّ أَوَّابٍ حَفِيظٍ[] مَنْ خَشِيَ الرَّحْمَنَ بِالْغَيْبِ وَجَاءَ بِقَلْبٍ مُنِيبٍ [ق: 31-33]

“And the Jannah (Paradise) will be brought close for Allah-fearing, no longer distant. (And it will be said,) .This is what you were promised for everyone who was oft-returning to Allah, vigilant (against sins). The one who feared the Rahmaan (The All-Merciful Allah), without seeing Him, and came up with a heart oriented towards Him.” (Qaaf 50:31-33)

The Prophet Hadhrat Muhammad (SallAllaho Alaihe WaSallam) said in an authentic Hadeeth:

"‏سبعة يظلهم الله في ظله يوم لا ظل إلا ظله‏:‏ إمام عادل، وشاب نشأ في عبادة الله ، ورجل قلبه معلق في المساجد، ورجلان تحابا في الله، اجتمعا عليه، وتفرقا عليه، ورجل دعته امرأة ذات منصب وجمال، فقال‏:‏ إنى أخاف الله، ورجل تصدق بصدقة فأخفاها حتى لا تعلم شماله ما تنفق يمينه، ورجل ذكر الله ‘خالياً ففاضت عيناه‏”

Seven people, Allah will give them His Shade on the Day when there would be no shade but the Shade of His Throne (i.e., on the Day of Resurrection).”

One of the seven mentioned is:“…..a man whom an extremely beautiful woman seduces (for illicit relation), but he (rejects this offer and) says: ‘I fear Allah’.” (Bukhari and Muslim)

Al-Ḥafiẓ Ibne Ḥajar 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.’ [15] Fatḥ al-Bāri’, vol. 2 p. 147

Fear of Allah (Subhaanahu Wa Ta’aalaa) 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. Allah (Subhaanahu Wa Ta’aalaa)’s Messenger (SallAllaho Alaihe WaSallam) said:

مَنْ خَافَ أَدْلَجَ وَمَنْ أَدْلَجَ بَلَغَ الْمَنْزِلَ

“Whoever fears, will travel during the night, and whoever travels during the night, will reach the destination.” (At-Tirmizi)

For this reason Al-Hafiz ʿUbaidullah bin Jaʿfar said: ‘A slave cannot seek aid for his religion with anything better than the fear of Allah.’ (Siyar al-A‘laam al-Nubulaa, vol. 6 p. 9)

The reality of this worship of the heart is borne across the limbs and thus we find in the aforementioned Hadeeth about the seven who will have Allah (Subhaanahu Wa Ta’aalaa)’s shade:
“A man whom an extremely beautiful woman seduces (for illicit relation), but he (rejects this offer and) says: ‘I fear Allah’.”

Disobedience presented itself before him in its most accomplished decorative appearance and most splendid manner of temptation, and though a man like any other man, nothing but the fear of Allah (Subhaanahu Wa Ta’aalaa) 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.” (Bukhari)

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 (Subhaanahu Wa Ta’aalaa) but his heart awoke and became replete with fear of Allah (Subhaanahu Wa Ta’aalaa), which prevented him from committing the sin.

The saying of Sa’eed bin Jubair is an exceptional definition of fear:
“[Real] fear means to fear Allah (Subhaanahu Wa Ta’aalaa) to the extent that your fear prevents your disobedience – that is fear.” (Ḥilyah al-Auliyaa, vol. 4 p. 276 and Siyar al-A‘laam al-Nubulaa, vol. 6 p. 9)