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Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
bayhaqi:6431 – Abū ʿAbdullāh al-Ḥāfiẓ > Abū al-Faḍl b. Ibrāhīm > Aḥmad b. Salamah > Isḥāq b. Ibrāhīm > al-Naḍr b. Shumayl > Mūsá b. Tharwān al-Muʿallim > Ṭalḥah b. ʿUbaydullāh b. Karīz al-Khuzāʿī > Um al-Dardāʾ > Sayyidī

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. (Using translation from Muslim 2732b)   

البيهقي:٦٤٣١ – أَخْبَرَنَا أَبُو عَبْدِ اللهِ الْحَافِظُ أنبأ أَبُو الْفَضْلِ بْنُ إِبْرَاهِيمَ ثنا أَحْمَدُ بْنُ سَلَمَةَ ثنا إِسْحَاقُ بْنُ إِبْرَاهِيمَ أنبأ النَّضْرُ بْنُ شُمَيْلٍ ثنا مُوسَى بْنُ ثَرْوَانَ الْمُعَلِّمُ حَدَّثَنِي طَلْحَةُ بْنُ عُبَيْدِ اللهِ بْنِ كَرِيزٍ الْخُزَاعِيُّ قَالَ حَدَّثَتْنِي أُمُّ الدَّرْدَاءِ قَالَتْ حَدَّثَنِي سَيِّدِي

أَنَّهُ سَمَعَ رَسُولَ اللهِ ﷺ يَقُولُ مَنْ دَعَا لِأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ  

رَوَاهُ مُسْلِمٌ فِي الصَّحِيحِ عَنْ إِسْحَاقَ بْنِ إِبْرَاهِيمَ

See similar narrations below:

Collected by Muslim, Suyūṭī

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

muslim:2732b – Isḥāq b. Ibrāhīm > al-Naḍr b. Shumayl > Mūsá b. Sarwān al-Muʿallim > Ṭalḥah b. ʿUbaydullāh b. Karīz > Um al-Dardāʾ from my husband

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.”  

مسلم:٢٧٣٢b – 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 ﷺ يَقُولُ: «مَنْ دَعَا لأَخِيهِ بِظَهْرِ الْغَيْبِ، قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ: آمِينَ وَلَكَ بِمِثْلٍ.»