Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "There are two supplications between which and Allah there is no veil: the supplication of the oppressed, and the supplication of a man for his brother in his absence."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11186