The Prophet ﷺ sent Muaz to Yemen and said to him, "Beware the supplication of the oppressed, for there is no barrier between it and Allah."
أن النبي ﷺ لبعث معاذا إلى اليمن، فقال: اتق دعوة المظلوم؛ فإنه ليس بينها وبين الله حجاب.
Bukhari narrated it in the Book of Injustices (2448), and Muslim in the Book of Faith (19), both from the hadith of Waki, who said, Zakariya bin Ishaq Makki told us, from Yahya bin Abdullah bin Sayfi, from Abu Mabad, the freedman of Ibn Abbas, from Ibn Abbas, and he related it. This is how Bukhari mentioned it in brief, and it was already mentioned in full in the Book of Faith.