The Companions, may Allah be pleased with them, and the righteous Salaf, (predecessors) were extremely fearful of Allah The Almighty. For instance, Ibn ‘Abbaas, may Allah be pleased with him, had two dark lines under his eyes due to the extent of his crying; ‘Umar, may Allah be pleased with him, once recited some verses which caused him to fall ill out of anxiety. Abu Bakr, may Allah be pleased with him, would say, “I wish I was a tree from which people eat (so that he would not to be held to account)”; ‘Uthmaan, may Allah be pleased with him, would say, “I wish that when I die I will not be resurrected” and Abu Hurayrah, may Allah be pleased with him, would faint three times whenever he would narrate the Hadeeth (narration) in which the Prophet, sallAllahu ‘alayhi wa sallam, informed us about the first three people who will be thrown into Hell;
The person who fears Allah The Almighty in this life, will be secure tomorrow in the Hereafter. When the believers are permitted into Paradise, fear will depart from them and their love of Allah The Almighty will increase. Imaam Ibn Rajab said, “Allah has created the creation in order to know Him and worship Him; He has provided them with evidences for His might and greatness in order for them to fear and glorify Him; He has described for them the severity of His punishment of those who disobey Him in order for them to avoid it by virtue of their good deeds (after the mercy of Allah).” This is the reason why Allah The Almighty repeatedly mentions Hell and the punishment He has prepared in it for those who disobey Him; He tells His slaves to fear Him, be conscious of Him, adhere to His commands, rush to fulfil what He has mandated upon them, do all that pleases Him, and refrain from that which He hates.
This is evident in the Sunnah (tradition) of the Prophet, sallAllahu ‘alayhi wa sallam, which is a clarification of the Quran. This is also apparent in the practice of our righteous Salaf (predecessors). If we ponder upon their condition, we will be amazed by the fear of Allah The Almighty and humility that they possessed, and how this caused them to elevate their ranks and exert extra efforts in the obedience of Allah The Almighty, as well as to easily refrain from minor disliked matters, to say nothing of prohibitions.
Fear of Allah The Almighty has different levels and ranks: the minimum level is that which makes a believer perform his Islamic obligations and refrain from prohibitions; if it exceeds this level then it promotes the desire to perform optional deeds and abstinence from minor disliked matters, and this is a praiseworthy type of fear. On the other hand, if it increases to the point that it makes one enter a state of despair which prevents him from exerting any effort, thinking that there is no use in doing so, and if it causes him grief or even kills him (as is the case in some instances), then this becomes a dispraised type of fear.
The desirable fear of Allah The Almighty drives a person to perform all his Islamic obligations and add optional deeds to them, besides causing him to refrain from prohibitions and minor disliked matters. There is a type of fear which is weaker than this which does not cause one to refrain from all prohibitions, or encourage one to perform all his Islamic obligations.
The more one knows about Allah The Almighty and his qualities and abilities, the more fearful he becomes, as Allah The Almighty described the angels by saying (what means): {“They fear their Lord above them…”}[Quran, 16: 50]
Allah The Almighty Says regarding His Messengers (what means): {“[Allah praises] those who convey the messages of Allah and fear Him and do not fear anyone but Allah…”}[Quran, 33: 39]
The fear of the devout slaves is greater because they seek a higher rank and fear that they may not attain it. What would one who is upon the straight path fear? He would fear his heart changing, as Allah The Almighty Says (what means): {“…And know that Allah intervenes between a man and his heart …”}[Quran 8: 24] He also fears having his rank reduced or decreased. Conversely, the sinner fears the evil consequences of his wicked deeds; his fear would benefit him only if it causes him to refrain from his evil deeds, regret, and repent.
Source: islamweb.net