رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 1495
The Book of Du'a (Supplications)
The Excellence of Supplicating in one's Absence
وَعَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَقُولُ: دَعْوَةُ الْمَرْءِ الْمُسْلِمِ لأَخِيهِ بِظَهْرِ الْغَيْبِ مُسْتَجَابَةٌ، عِنْدَ رَأْسِهِ مَلَكٌ مُوَكَّلٌ كُلَّمَا دَعَا لأَخِيهِ بِخَيْرٍ قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ: آمِينَ، وَلَكَ بِمِثْلٍ ((رَوَاهُ مُسْلِم)).
Abud-Darda' (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "The supplication of a Muslim for his (Muslim) brother in his absence will certainly be answered. Everytime he makes a supplication for good for his brother, the angel appointed for this particular task says: 'A meen! May it be for you, too'." [Muslim].
Riyad as-Salihin, 1495