Do not belittle the power of du'ah

"Do you make fun of du’ah and belittle it while you do not know what it has the power to do!? The arrow of the night does not miss its target; it has an appointed time and the appointed time has its ending. So if my Lord wishes, He holds it back. And if the decree is to be fulfilled, He sets it forth." 

Imaam ash Shafi’i - [ad-Diwan]