It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: When any one of you makes supplication, he should do so with certainty and firmness, and he should not say: O Allah! If You wish, grant this to me, because there is no one who can compel Allah Almighty.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11980