ENOfficial
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: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا النَّضْرُ بْنُ شُمَيْلٍ، حَدَّثَنَا مُوسَى بْنُ سَرْوَانَ، الْمُعَلِّمُ حَدَّثَنِي طَلْحَةُ بْنُ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ، قَالَ حَدَّثَتْنِي أُمُّ الدَّرْدَاءِ، قَالَتْ حَدَّثَنِي سَيِّدِي، أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " مَنْ دَعَا لأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ " .
sunnah.com