Sahih Muslim Book 035, Hadith Number 6479.

Chapter : Supplication should be made with conviction and will.

Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: None of you should say to Allah (like this): O Allah, grant me mercy, if thou so likest. The supplication (of his) should (be permeated with) conviction (that it would be accepted by the Lord), for Allah is the Doer of (everything) He likes to do, and there is none to force Him (to do or not to do this or that).