The Prophet ﷺ sent Muaz to Yemen and said to him, "Beware of the supplication of the wronged, for there is no barrier between it and Allah."
أن النبي ﷺ بعث معاذا إلى اليمن فقال: اتق دعوة المظلوم؛ فإنها ليس بينها وبين الله حجاب.
Bukhari related it in the Book of Injustices (2448), from Yahya bin Musa, from Waki, from Zakariya bin Ishaq Makki, from Yahya bin Abdullah bin Sayfi, from Abu Mabad the freed slave of Ibn Abbas, from Ibn Abbas, and so he mentioned it.Muslim related it in the Book of Faith (19:29) through channels from Waki with the same wording, at greater length.Abu Bakr bin Abu Shayba (the teacher of Muslim) said, Waki sometimes said, from Ibn Abbas, that Muaz said, "The Messenger of Allah sent me ﷺ