Ibn Abbas reported: The Messenger of Allah, peace and blessings be upon him, said, “Two supplications have no barrier between them and Allah: the supplication of the oppressed, and the supplication of a man for his brother behind his back.”
Source: al-Mu’jam al-Kabīr 11232
Grade: Sahih (authentic) according to Al-Suyuti
عَنِ ابْنِ عَبَّاسٍ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ دَعْوَتَانِ لَيْسَ بَيْنَهُمَا وَبَيْنَ اللَّهِ حِجَابٌ دَعْوَةُ الْمَظْلُومِ وَدَعْوَةُ الْمَرْءِ لِأَخِيهِ بِظَهْرِ الْغَيْبِ
11232 المعجم الكبير للطبراني
4191 المحدث السيوطي خلاصة حكم المحدث صحيح في الجامع الصغير