Sayyiduna Abu Huraira (may Allah be pleased with him) narrates: The Noble Prophet (peace and blessings be upon him) said: "The supplication of the oppressed is raised above the clouds and the gates of the heavens are opened for it, and the Lord says: By My honor, I will surely help you, even if it is after a while."
Imam Ibn Hibban (may Allah have mercy on him) says: Abu al-Mudallah's name is Ubaydullah Madani; he is trustworthy.