the Messenger of Allah ﷺ said, “There is no believing servant who supplicates for his brother behind his back except that the Angels say: May the same be for you.”  

قَالَ رَسُولُ اللَّهِ ﷺ «مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ.»  

that he heard the Messenger of Allah ﷺ say, “He who supplicates for his brother behind his back (in his absence), the representative Angel [assigned] to him says: Amen, and it is for you also.”  

أَنَّهُ سَمِعَ رَسُولَ اللَّهِ ﷺ يَقُولُ: «مَنْ دَعَا لأَخِيهِ بِظَهْرِ الْغَيْبِ، قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ: آمِينَ وَلَكَ بِمِثْلٍ.»  

I visited Abu Darda's house in Syria. I did not find him there but Umm Darda' (was present at the house). She said: Do you intend to perform Hajj during this year? I said: Yes. She said: Do supplicate Allah for blessings upon us...

for Allah's Apostle ﷺ used to say: The supplication of a Muslim for his brother in his absence is responded to. An angel is commissioned next to him. Every time he prays for good for his brother, the commissioned Angel says: Āmīn, and says: May it be for you too.

I then went to the bazar and met Abū Dardāʾ and he narrated like this from Prophet ﷺ.  

قَدِمْتُ الشَّامَ فَأَتَيْتُ أَبَا الدَّرْدَاءِ فِي مَنْزِلِهِ فَلَمْ أَجِدْهُ وَوَجَدْتُ أُمَّ الدَّرْدَاءِ فَقَالَتْ أَتُرِيدُ الْحَجَّ الْعَامَ فَقُلْتُ نَعَمْ قَالَتْ فَادْعُ اللَّهَ لَنَا بِخَيْرٍ

فَإِنَّ النَّبِيَّ ﷺ كَانَ يَقُولُ «دَعْوَةُ الْمَرْءِ الْمُسْلِمِ لأَخِيهِ بِظَهْرِ الْغَيْبِ مُسْتَجَابَةٌ عِنْدَ رَأْسِهِ مَلَكٌ مُوَكَّلٌ كُلَّمَا دَعَا لأَخِيهِ بِخَيْرٍ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ۔»

قَالَ فَخَرَجْتُ إِلَى السُّوقِ فَلَقِيتُ أَبَا الدَّرْدَاءِ فَقَالَ لِي مِثْلَ ذَلِكَ يَرْوِيهِ عَنِ النَّبِيِّ ﷺ