Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
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
abudawud:1534 – Rajāʾ b. al-Murajjá > al-Naḍr b. Shumayl > Mūsá b. Tharwān > Ṭalḥah b. ʿUbaydullāh b. Karīz > Um al-Dardāʾ > Sayyidī Abū al-Dardāʾ

I heard the Messenger of Allah ﷺ say: When a Muslim supplicates for his absent brother the angels say: Amin, and may you receive the like.  

أبو داود:١٥٣٤ – حَدَّثَنَا رَجَاءُ بْنُ الْمُرَجَّى حَدَّثَنَا النَّضْرُ بْنُ شُمَيْلٍ أَخْبَرَنَا مُوسَى بْنُ ثَرْوَانَ حَدَّثَنِي طَلْحَةُ بْنُ عُبَيْدِ اللَّهِ بْنِ كَرِيزٍ حَدَّثَتْنِي أُمُّ الدَّرْدَاءِ قَالَتْ حَدَّثَنِي سَيِّدِي أَبُو الدَّرْدَاءِ

أَنَّهُ سَمِعَ رَسُولَ اللَّهِ ﷺ يَقُولُ إِذَا دَعَا الرَّجُلُ لأَخِيهِ بِظَهْرِ الْغَيْبِ قَالَتِ الْمَلاَئِكَةُ آمِينَ وَلَكَ بِمِثْلٍ  

See similar narrations below:

Collected by Muslim

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.»