Maliks Muwatta Book 15, Hadith Number 29.

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 answered as long as you are not impatient and say, ‘I have made a dua and I have not been answered.'”