Full Sharh — Riyad as-Salihin 816

English Translation

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'.".