كتاب الذكر والدعاء والتوبة والاستغفار
48
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness
(23)
Chapter: The Virtue Of Praying For The Muslims In Their Absence
(23)
باب فَضْلِ الدُّعَاءِ لِلْمُسْلِمِينَ بِظَهْرِ الْغَيْبِ ‏‏
Sahih Muslim 2733, 2732 c

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).
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، أَخْبَرَنَا عِيسَى بْنُ يُونُسَ، حَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ أَبِي، سُلَيْمَانَ عَنْ أَبِي الزُّبَيْرِ، عَنْ صَفْوَانَ، - وَهُوَ ابْنُ عَبْدِ اللَّهِ بْنِ صَفْوَانَ - وَكَانَتْ تَحْتَهُ الدَّرْدَاءُ قَالَ 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 صلى الله عليه وسلم 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 صلى الله عليه وسلم ‏.‏

Reference : Sahih Muslim 2733, 2732 c
In-book reference : Book 48, Hadith 121
USC-MSA web (English) reference : Book 35, Hadith 6590
  (deprecated numbering scheme)