The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
Ṣaḥīḥ Muslim 6930
دَعْوَةُ الْمَرْءِ الْمُسْلِمِ لأَخِيهِ بِظَهْرِ الْغَيْبِ مُسْتَجَابَةٌ عِنْدَ رَأْسِهِ مَلَكٌ مُوَكَّلٌ كُلَّمَا دَعَا لأَخِيهِ بِخَيْرٍ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ آمِينَ وَلَكَ بِمِثْلٍ
daʿwah al-marʾi al-muslimi lʾakhīhi biẓahri al-ghaybi mustajābatun ʿinda raʾsihi malakun muwakkalun kullamā daʿā lʾakhīhi bikhayrin qāla al-malaku al-muwakkalu bihi āmīna walaka bimithlin
Transliteration generated from the Arabic vowel marks
The full narration
Safwan (and he was Ibn 'Abdullah b. Safwan, and he had been married to Umm Darda') reported: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 (may peace be upon hiin) used to say: The supplication of a Muslim for his brother at his back (in his absence) is responded so long as he makes a supplica- tion for blessings for his brother and the commissioned Angel says: Amen, and says: May it be for you too I I went to the bazar and met Abfi Dardi' and he narrated like this from Allah's Messenger (ﷺ)
Source
Ṣaḥīḥ Muslim · 6930