It is narrated from Sayyiduna Khuzaymah bin Thabit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Beware of the supplication of the oppressed, for it is carried upon the clouds. Allah, Exalted be His Majesty, says: By My Might and My Majesty, I will surely help you, even if it be after a while."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3703