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»
The Messenger of Allah (sallallahu alayhi wa sallam) 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’.”.
Riyad as-Salihin 1495, Book 17, The Book of Du'a (Supplications), Hadith 31. 1 min read