THE REALITY OF FEAR
Know that fear is a pain and burning in the heart caused by the anticipation of receiving harm in the future. The example of it is like a person who has committed a crime against a king, and then fell into his hands. So he would fear being killed, while considering the possibility that he may be pardoned. But the pain in his heart would he according to:
1. The level of his knowledge of the cause that would lead to his execution
2. The enormity of his crime
3. Its impact upon the king
The weaker these causes are, the less he would fear. It may also be that fear be caused by other than a crime. It could be based on (knowing) the attributes of the one who is feared, his majesty and greatness. One may learn that if Allāh were to destroy the heavens and earth, he would not have concern, and nothing could stop Him from that. Thus a person's fear is according to his realisation of his own faults, and the greatness of Allāh and His freedom from any need, and that He is not asked about what He does.
The most fearful of people is the one who is most knowledgeable of himself and his Lord, and this is why the Prophet (ﷺ) said, "I am the most knowledgeable of you regarding Allāh, and the one who fears Him the most among you." *
* Bukhārī, #4776 and Muslim, #1401
Book: Ranks Of The Fearful ('Al-Khawf Haqiqatihi Wa Bayan Darajatihi')
By Shaykh al-Islām Muwaffaq al-Dīn Ibn Qudāmah al-Maqdīsī (d. 620 AH)
Translated by Yousef Hussin
Dār as-Sunnah Publishers
P. 18, 19