The Prophet gave glad tidings to a man who was inspired with du’a so that he would be among those who would receive the Mercy of Allah. He (the Prophet) said:
“If the gates of du’a are opened by man, the gates of mercy will be opened before him. And the most beloved thing for which Allah loves to be asked is safety.” [Tirmidhi and Hakim]
The supplicant also received glad tidings from the Prophet that Allah would keep him safe and he would have the privilege of du’a being a weapon to fight the enemies and defend himself with:
“Du’a is a weapon of a believer, a pillar of religion, and a light of the
heavens and the Earth.” [Hakim]
The Prophet also gave glad tidings to the person who supplicates to Allah so that his supplication would be answered and that his approach towards Allah would be accepted.
Allah is Ever-Merciful and Ever-Generous to the extent that when man lifts his hands skyward (invoking Allah), He dislikes replying without rewarding him.
The Prophet explained also how the answer for du’a can be achieved. The answer, whether spontaneous or delayed, is good for the supplicant. Thus his du’a is ultimately beneficial for him. The Prophet said:
“If a Muslim invokes Allah I for other than sin or the severing of relations, Allah I in turn responds to his Du’a in one of three ways; either He accepts it (and grants what was asked for),or He delays its reward to the Hereafter, or He removes a calamity from him that equals his Du’a (in magnitude). The Companions said, “ We will then supplicate often.” The Prophet r replied, “Allah I will reward you greatly.” [Ahmad, Bazzar, Abu Ya’la and Hakim]
Du’a prevents calamities and lightens Allah’s will through Allah’s will. The Prophet said:
“Precaution can never prevent predestination. Du’a surely avails in regards to what has already taken place (in terms of seeking forgiveness) and what has not yet come to pass (in terms of asking for favors). A calamity may have descended from heaven when du’a meets it. Consequently, a confrontation between them will take place until the Day of Judgement.” [Bazzar, Tibarani and Hakim]
He also said, “Nothing prevents predestination except du’a, and nothing lengthens age except righteousness.” [Tirmidhi]
The Prophet guided us to the way of answering du’a, i.e., continuous request to Allah in every time. He said, “Whoever desires to have his du’a accepted by Allah in adversity, should make frequent du’a in prosperity.” [Tirmidhi and Hakim]
“Nothing is more beloved to Allah than du’a in prosperity.” [Tirmidhi, Ibn Majah, Ibn Hibban and Hakim]
These ahadith and others indicate that du’a is one of the most powerful means of preventing a calamity, du’a is an enemy to calamity. It confronts, prevents, or weakens the calamity as mentioned in the previous hadith which indicated that du’a has three stages with calamity:
1. Du’a may be stronger than calamity so it prevents its occurrence.
2. Du’a may be weaker than a calamity, then the latter would occur, but Du’a may weaken its effects.
3. A confrontation between both may take place until each one prevents the other.
However, Du’a may miss the answer for the following reasons:
i. It may be weak, e.g. it includes something that is abhorred by Allah such as aggression.
ii. The unconsciousness and inattentiveness of the heart at the time of du’a. In this case, it would be like an arrow which is shot from a soft bow. It would never reach the target. Hakim reported, in his Mustadrak, Abu Hurairah who said the Prophet said, “Invoke Allah with full conviction that He will accept your invocation and know that Allah never accepts the invocation of a heedless heart.”
iii. Consuming of Haram (Unlawful), committing sins, the indulgence in prohibited amusement and the hardened heart that shies from remembering Allah.
Therefore, du’a is a useful remedy for illness but the heedlessness of one’s heart turns it null and void.
Ref.: All about du’as and supplications.