The Messenger of Allah ﷺ sent Muaz bin Jabal to Yemen and said to him, "Beware of the supplication of the wronged, for there is no barrier between it and Allah."
حدثنا أبو كريب حدثنا وكيع عن زكريا بن إسحق عن يحيى بن عبد الله بن صيفي عن أبي معبد
أن رسول الله ﷺ بعث معاذ بن جبل إلى اليمن فقال اتق دعوة المظلوم فإنها ليس بينها وبين الله حجاب
SourcingBukhari also narrated it, in the chapter on zakat, section on the obligation of zakat, hadith no. 1395, from the hadith of Zakariya bin Ishaq, with the same wording.On this topic, there are also narrations from Anas [Ahmad: 3/153], Abu Hurairah [mentioned previously: 1905], and Abdullah bin Amr, though I could not locate it, as well as Abdullah bin Umar [Hakim: 1/29] and Abu Saeed [Bukhari, in Tarikh al-Kabir: 7/139].