Du’aa’ is one of the most beneficial of remedies; it is the enemy of calamity; it wards it off, remedies it, prevents it befalling, alleviates it or reduces it if it befalls. It is the weapon of the believer. In the case of calamity, one of three scenarios must apply:
1. It (du’aa’) is stronger than the calamity, so it wards it off;
2. It is weaker than the calamity, so the calamity overpowers it and befalls the person, but it may reduce it even if it is weak;
3. They resist one another and each prevents the other.
Cited in al-Jawaab al-Kaafi by Ibn al Qayyim