It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) sent Sayyiduna Mu'adh (may Allah be pleased with him) to Yemen (and he narrated the hadith). At its end he said: "Beware of the supplication of the oppressed, for there is no veil between his supplication and Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العارية / 11502