Yahya related to me from Malik from Abu'z Zinad from al-Arajfrom Abu Hurayra that the Messenger of Allah, may Allah bless him andgrant him peace, said, ''When you are making dua do not say; 'O Allah,forgive me if You wish. O Allah, forgive me if you wish.' You shouldbe firm in your asking, for there is no compelling Him."
USC-MSA web (English) reference: Book 15, Hadith 28
Arabic reference: Book 15, Hadith 500