عن ابن عباس رضي الله عنهما أن النبي صلى الله عليه وسلم بعث معاذا إلى اليمن فقال: اتق دعوة المظلوم فإنها ليس بينها وبين الله حجاب (صحيح البخاري الرقم: 2448)

Hazrat ‘Abdullah bin ‘Abbaas (radhiyallahu ‘anhuma) reports that when Rasulullah (sallallahu ‘alaihi wasallam) dispatched Hazrat Mu‘aaz bin Jabal (radhiyallahu ‘anhu) to Yemen, he said to him, “Beware of the du‘aa of the oppressed person, for indeed there is no barrier between the du‘aa of the oppressed and Allah Ta‘ala (i.e. Allah Ta‘ala accepts the du‘aa of the oppressed person very swiftly).”