Yahya related to me from Malik from Ibn Shihab from Abu Ubayd,the mawla of Ibn Azhar, from Abu Hurayra that the Messenger of Allah,may Allah bless him and grant him peace, said, "You will be answeredas long as you are not impatient and say, 'I have made a dua and Ihave not been answered .' "
USC-MSA web (English) reference: Book 15, Hadith 29
Arabic reference: Book 15, Hadith 501