Angels make duʿāʾ for the one who makes duʿāʾ for this brother
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.»