Sahih Al Bukhari – Book of Invocations
Volumn 008, Book 075, Hadith Number 350.
—————————————–
Narated By Anas : Allah's Apostle said, "When anyone of you appeal to Allah for something, he should ask with determination and should not say, 'O Allah, if You wish, give me.', for nobody can force Allah to do something against His Will.
