Narrated Abu Huraira (may Allah be pleased with him): The Messenger of Allah (peace and blessings of Allah be upon him) said: The supplication of a servant is accepted as long as he does not become hasty and say, "I made supplication but it was not accepted."
Hadith Takhrij«صحیح، أخرجه البخاري في: 80 كتاب الدعوات: 22 باب يستجاب للعبد ما لم يعجل»
Brief Explanation
It is not correct to be hasty regarding the acceptance of supplication (dua). If the supplication is made with sincerity of heart and the conditions and etiquettes of supplication are observed, then it will certainly be accepted sooner or later. Even if it does not appear to be accepted outwardly, it will become a treasure for the Hereafter. (Raaz)