Translation: Ishaaq b. AbdurRaheem Aboo Aamir
Duaa (invoking Allaah for our needs) is from among the most beneficial healings. It is indeed an enemy to trials, it wards off trials and gets them solved (when they occur), it also prevents and removes tribulations, or reduces their effect when they land. Duaa is a weapon of the believer.
There are three things the Duaa do with trials:
1. It may be that it is stronger therefore preventing the trial from occurring.
2. It may be that it is weaker than the trial thus the latter overwhelming it and consequently the slave will be affected by the trial though the Duaa will lessen the effect of (the stronger) trial despite being weak.
3. That they are equal such that each of them blocks the other.
Al-Haakim recorded in his mustadarak on the authority of Aaisha that she said she heard the Messenger of Allaah - sallallaahu alahyi wa sallam - saying: 'Being careful cannot prevent Qadar. Duaa benefits in what has happened and what has not. A trial might descend and Duaa will meet it while both will remain in struggle till the Day of Resurrection.' (Declared hasan in Saheeh Jaami 6/241).
Also on the authority of Ibn Umar, the Prophet - sallallaahu alahyi wa sallam - said: 'Duaa benefits in what has happened and what has not, upon you is Duaa O slaves of Allaah.' (Declared hasan in Saheeh Jaami, 3409).
Also in the hadith recorded on the authority of Thaubaan, the Prophet - sallallaahu alahyi wa sallam - said, 'Nothing can prevent a preordainment (from occurring) except Duaa, nothing increases the lifespan except piety, and a man would be deprived of sustenance due to a sin he committed.' (The hadeeth is hasan with the combination of its various routes).
Ad-Duaa wa ad-Dawaa, pp. 10-12, Daar Ibn Jawzee Edn, 1423AH