The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
حَدَّثَنِي أَحْمَدُ بْنُ عُمَرَ بْنِ حَفْصٍ الْوَكِيعِيُّ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ، حَدَّثَنَا أَبِي، عَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ، عَنْ أُمِّ الدَّرْدَاءِ، عَنْ أَبِي الدَّرْدَاءِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ " .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
From Sahih Muslim, whose reports the scholars accept as authentic.