The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
Ṣaḥīḥ Muslim 2732b
Say this
مَنْ دَعَا لأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ
man daʿā lʾakhīhi biẓahri al-ghaybi qāla al-malaku al-muwakkalu bihi āmīna walaka bimithlin
Transliteration generated from the Arabic vowel marks
The full narration
Umm Darda' reported:My husband reported that he heard Allah's Mes- senger (ﷺ) as saying: He who supplicates for his brother behind his back (in his absence), the Angel commissioned (for carrying supplication to his Lord) says: Amen, and it is for you also
Source
Ṣaḥīḥ Muslim · 2732b
Ṣaḥīḥ