The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
Ṣaḥīḥ Muslim 6927
مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ
mā min ʿabdin muslimin yadʿū lʾakhīhi biẓahri al-ghaybi illā qāla al-malaku walaka bimithlin
Transliteration generated from the Arabic vowel marks
The full narration
Abu Dharr reported that Allah's Messenger (ﷺ) said:There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do not say: The same be for you too
Source
Ṣaḥīḥ Muslim · 6927
Ṣaḥīḥ