Riyad as-Salihin, Book 17, Hadith 1495 (In-Book) ⓘ

Abud-Darda' (May Allah be pleased with him) reported: The Messenger of Allah 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].

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)‏‏)‏‏.‏