Allah, the Wise, has said:
وَادْعُوْهُ خَوْفاً وَّ طَمَعاً
"And call on Him fearing and hoping." 
The Noble Prophet (sa) has said:
أَتَمُّکُمْ عَقْلاً أَشَدُّکُمْ خَوْفاً
"The most perfect and complete from amongst you in intellect is the one who is most fearful amongst you (of Allah)." 
Fear of Allah is the sentinel of the heart; this is because a fearful person, by means of fear, remains mindful of Divine pleasure and soars to lofty heights. He witnesses the Divine threats and warnings, and thus refrains from deeds that are dictated by carnal and base desires.
A person who worships Allah out of His fear never gets deviated and eventually reaches his goal and objective. How can he afford not to be fearful especially since he does not possess knowledge of what his final outcome would be and is unaware if his Book of Deeds would be light or heavy?
A fearful person finds himself torn between two fears - fear of the past and that of the future. Fear serves to suppress one's (reckless) soul and when a person's soul is suppressed with respect to carnal and capricious desires, his heart comes to life. This leads to steadfastness, which eventually prepares the ground for the heart to develop hope and become hopeful (of divine mercy).
1 – The Fearful Youth
Salman Farsi was passing through the blacksmiths' market of Kufah when he observed that a crowd had gathered around a youth who lay on the ground, senseless. When the people saw Salman, they requested him to recite a supplication so that the youth could come out of his unconsciousness.
As soon as Salman came closer, the youth got up and said: "There is nothing wrong with me. It is just that I was passing through this market when I noticed the blacksmiths striking metal with their iron hammer and this made me recall what Allah has said in the Qur'an: "In addition there will be maces of iron (to punish) them.") As this verse crossed my mind, I was overcome by this state."
Salman became interested in the youth, grew fond of him and made him his brother. They were always friends with each other till one day the youth fell ill and was almost on the verge of dying. Salman sat down near his head and then, addressing Izra'eel, said: "O' Izra'eel! Be tolerant and lenient towards my young brother and be kind and gentle to him!"
"O' Servant of Allah! I am the friend of all the believers and kind to them all," Izra`il replied.
2 – The Mute Language of the Stone
It has been reported that once in the course of his journey, one of the Prophets came across a small stone and observed that a large quantity of water flowed out from it. As the incident left him greatly astonished, Allah made speech come out from the stone, which said: "Ever since I heard that men and stones would be the fuel of the fire of Hell, I have been in a state of weeping (out of fear that I should be one of those stones)."
The stone then requested the Prophet to pray that it should remain protected from the fire (of Hell) and the Prophet, acceding to his request, prayed for it.
It so happened that after a period, the Prophet again passed by that place and, witnessing that the water still continued to flow from the stone as before, inquired: "What is it that makes you weep now?"
The stone replied, "Before I could be satisfied of my deliverance from the fire (of Hell), my tears were out of fear, but now, I weep out of thanksgiving and due to joy and happiness."
3 – Punishment by Fire
Once, Amirul Mo'minin (as) was in the company of his companions when a person approached him and said: "O' Amirul Mo'minin (as)! Purify me for I have committed sodomy with a boy."
"Go home for you appear to have been affected by bile or melancholia," advised the Imam (as).
The next day the person turned up again and confessed to his dirty act but the Imam (as) repeated what he had previously stated. The third day too he confessed and again Imam Ali (as) repeated his earlier advice. When the man arrived on the fourth day and confessed to his crime, the Imam (as) said: "Now that you have confessed four times, choose one of the three punishments, which the Noble Prophet (sa) has stipulated for this act - to be beheaded by means of a sword, to be hurled down from a height or to be burned with your hands and feet tied."
"Which of these three punishments would be the severest for me?" the man asked.
"Burning by fire."
"O' Ali! (as), I choose this punishment."
The Imam (as) told him to prepare for it. The man got up, offered a two-rak'at prayer and supplicated: "O' Lord! I have committed sin and You are aware of it. Fearing Your wrath, I have approached the successor and the cousin of the Noble Prophet (sa) and have asked him to purify me of it. He asked me to choose one of the three punishments and I have chosen the most severe one. O' Lord! I plead to You by Your mercy to make my burning in this world an expiation for my sin and not to burn me in the Hereafter!"
Having said this, he got up, began to weep and then hurled himself into the pit of roaring flames. The Imam (as) began to weep when he saw this and so did his companions; then he said in a loud voice: "O' Man! Rise from within the fire for you have caused the angels to weep. Allah has accepted your repentance. Rise and henceforth do not conduct such an act!"
It is reported in another tradition that a person asked Ali (as), "O' Amirul Mo'minin (as)! Do you invalidate the punishment of Allah?"
Imam Ali (as) replied, "Woe unto you! Whenever there exists an Imam appointed by Allah and a sinner repents for his sin, it is upon Allah to forgive him."
4 – The Fearful Ones
When the verse: "And surely Hell is the promised place of them all. It has seven gates; for every gate there shall be a separate party of them") was revealed to the Noble Prophet (sa), he wept so intensely that it made his companions weep too, but none knew what it was that Jibra`il had revealed which had made the Noble Prophet (sa) wail in such a fashion.
One of the companions went to Fatimah and informed her of the incident. Covering herself with her cloak, which was patched in twelve places by means of date palm leaves, she came out of the house. When Salman Farsi's eyes fell upon the cloak, he looked at it in amazement and then, bursting into tears, said to himself: "The emperors of Rome and Iran attire themselves in silken and gold-embroidered clothes but the daughter of the Noble Prophet (sa) covers herself with such a cloak!"
When Fatimah came to the Noble Prophet (sa), he said to Salman: "My daughter belongs to a group that has taken great precedence in subservience and obedience (to Allah)."
Fatimah then asked, "Father! What was it that made you grieve?"
The Noble Prophet (sa) recited the verses which had been brought by Jibra`il. The mention of Hell and the punishment of the fire left her so distraught that her knees failed to hold her weight and she collapsed onto the floor saying, "Woe unto he who enters the fire."
"O' I wish I had been a sheep that was eaten and my skin had been torn apart so that I would never have had to hear of the fire of Hell," uttered Salman.
"O' I wish my mother had never given birth to me so that I would never have heard of the Hell-fire," said Abu Dharr.
"I wish I was a bird in the desert so that I would not have had Reckoning nor punishment, and would not have heard of the fire of Hell," muttered Miqdad.
Amirul Mo'minin (as) said, "I wish the wild animals had torn me into pieces and my mother had not given birth to me so that I would not have had to hear of the Hell-fire." Then, placing his hand upon his head, he began to cry and wailed, "Oh! How distant is the journey of the Day of Judgment! Woe to those who did not make provision for the Day of Judgement. In this journey of the Day of Judgment, they shall be led towards the fire; O' the sick ones, who shall be in the binds of captivity and whose injuries shall never be treated. None shall step forward to untie them; fire shall be their food and water, and they shall be turned upside down in the various stations of Hell."
5 – Yahya
When Prophet Yahya (as) observed the clerics of Baytul Maqdas wearing veils made of haircloth and headgears of cotton, he requested his mother to make a similar dress for him. Later, he began worshipping with them in Baytul Maqdas.
One day, Yahya (as) looked at his body, which had become considerably thin, and began to weep. Allah revealed to him: "You cry over your body that has thinned down? By My Glory and Majesty! Had you possessed the slightest knowledge of the fire (of Hell), you would have worn overcoats made of iron, not these woven clothes." Hearing this, Yahya wept to such an extent that the flesh of his cheeks became worn out.
Zakariyya (as) said to his son, "I had sought you from Allah so that you could be the apple of my eyes. Why do you behave in such a manner?"
"Father, but was it not you who had said: 'Surely, between Paradise and Hell there lies a pass and none except those who cry immensely due to fear of Allah, shall be able to traverse it'," Yahya replied.
"Yes, I did!," admitted Zakariyya (as).
Whenever Zakariyya (as) intended to preach and exhort the Bani Isra`il, he would first look around him, and if he noticed Yahya (as) amongst them, he would refrain from mentioning anything about Paradise and Hell. Once, Zakariyya (as) was engaged in delivering a sermon to the people when Yahya (as), his head covered with his cloak, arrived and sat down amongst the people. Zakariyya (as), who had not noticed Yahya (as), preached: "Allah has said: 'In Hell there is a mountain by the name of Sakaran alongside which there lies a desert by the name of Ghadhaban, in which there is a well whose depth is equivalent to one hundred years of travel. Within this well, there exist caskets of fire and within these caskets lie chests of fire, which in turn contain clothes and chains of fire."
As soon as Yahya (as) heard the name 'Sakaran', he raised his head, shrieked and in a state of utter distress and disturbance, rushed out and headed towards the wilderness.
Zakariyya and Yahya's mother set off in search of him; some of the youths of Bani Isra`il too, out of respect for Yahya's mother joined them in their search. They came across a shepherd and asked him if he had seen a youth with Yahya's description.
"Are you looking for Yahya Ibn Zakariyya?" inquired the shepherd.
"Yes," they replied.
"He is presently in a particular place with his feet in the water and his eyes glued towards the heavens, praying and communicating with his Lord," he explained.
The search party went to that place and located him. Calling Yahya (as) towards herself, his mother placed him under the oath of Allah and requested him to return home. Soon, Yahya (as) returned home with his mother.
 Noble Qur'an, Suratul ھaraf (7), Verse 56
 Jamea al-Saaadat, Volume 1, Page 225
 Tadhkeratul Haqaiq, Page 83
 لَهُم مَقاَمِعُ مِن حَدِيد (Noble Qur'an, Suratul Hajj (22), Verse 21
 Dastan-e-Jawanan, Page 94
 Shanidani-ha-e Tarikh, Page 388; Mahajjatul Baiza, Volume 7, Page 142
 Dastan-ha-e-Zindagi-e-Ali (as), Page 51; Qadhawat-ha-e-Muhayyir al-aUqul
 وَ اِنَّ جَهَنَّمَ لَمَوعِدُهُم اَجمَعِينَ لهَاَ سَبعَةُ اَبواَبٍ لِکُلَّ بَابٍ مِنهُم جُزْءٌ مَقسُومٌ (Noble Qur'an, Suratul Hijr (15), Verse 43-44
 Pand-e-Tarikh, Volume 4, Page 221; Biharul Anwar, Volume 10, Page 26
 Risalah Liqaullah, Page 157-164; Amali of al-Saduq