The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Muslim :: Book 35 : Hadith 6593
Abu Huraira reported that Allah's Messenger (may peace be upon him) said: The supplication of every one of you is granted if he does not grow impatient and says: I supplicated but it was not granted.