It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "There are three people whose supplication is never rejected: the just imam; the fasting person until he breaks his fast; and the supplication (against oppression) of the wronged person, which is carried up upon the clouds, the gates of the heavens are opened for it, and Allah Almighty says: 'By My Might! I will surely help you, even if after a while.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16648