Sahih Muslim Book 035, Hadith Number 6478.

Chapter : Supplication should be made with conviction and will.

Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: When one of you makes a supplication (to his Lord) one should not say: O Allah, grant me pardon, if Thou so likest, but one should beg one's (Lord) with a will and full devotion, for there is nothing so great in the eye of Allah which He cannot grant.