"The Messenger of Allah sent Mu'adh [bin Jabal] to Yemen, and said: 'Beware of the supplication of the oppressed; for indeed there is no barrier between it and Allah.'"  

أَنَّ رَسُولَ اللَّهِ ﷺ بَعَثَ مُعَاذَ بْنَ جَبَلٍ إِلَى الْيَمَنِ فَقَالَ اتَّقِ دَعْوَةَ الْمَظْلُومِ فَإِنَّهَا لَيْسَ بَيْنَهَا وَبَيْنَ اللَّهِ حِجَابٌ