Compiled by Imam Yahya ibn Sharaf al-Nawawi
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)).
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].