Hadith #816
Ungraded
Chapter: The Book of Du'a (Supplications)
كتاب الدعوات
Arabic Text
وعنه أن رسول الله صلى الله عليه وسلم كان يقول: "دعوة المرء المسلم لأخيه بظهر الغيب مستجابة، عند رأسه ملك موكل كلما دعا لأخيه بخير قال الملك الموكل به: آمين، ولك بمثل" ((رواه مسلم)).
English Translation
Abud-Darda' (may Allah be pleased with him) reported that 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'.".
Quran References
Reference: Riyadh al-Salihin, Book 16, Hadith 816
Sunnah.com ref: riyadh-salihin 816
Islam.wiki ID: IWH-67654