The supplication of the oppressed

عَنِ ابْنِ عَبَّاسٍ رَضِیَ اللہُ عَنْہُ أَنَّ رَسُوْلَ اللہِ صَلَّی اللہُ عَلَیْہِ وَسَلَّمَ بَعَثَ مُعَاذَ بْنَ جَبَلٍ اِلَی الْیَمَنِ فَقَالَ اتَّقِ دَعْوَۃَ الْمَظْلُوْمِ فَاِنَّھَا لَیْسَ بَیْنَھَا وَبَیْنَ اللہِ حِجَابٌ

Hazrat Ibn Abbas^ra narrated, “The Holy Prophet, peace and blessings of Allah be upon him, sent Muaz bin Jabal to Yemen, and said, ‘Beware of the supplication of the oppressed, for indeed, there is no barrier between it and Allah.’”

(Jami‘ al-Tirmidhi, Kitab al-Bir wa al-Sila an Rasool Allah)