Maliks Muwatta Book 15, Hadith Number 28.
Section : Dua (Supplication).
Yahya related to me from Malik from Abu’z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant 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 should be firm in your asking, for there is no compelling Him.”