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].
App reference: Book 17, Hadith 31
Arabic/English book reference: Book 17, Hadith 1495