1495 – From him also: the Messenger of Allah, peace and blessings be upon him, used to say, “The supplication a Muslim offers for his brother in his absence is answered. At his head stands an angel entrusted with him, and whenever he prays for his brother for something good, that angel says, ‘Āmīn, and the same for you.’” Narrated by Muslim.

[Commentary]
The author next cites the ḥadīth of Abū al-Dardāʾ, may Allah be pleased with him, in its two wordings: when a person prays for his brother in his absence, the angel says, “Āmīn, and the same for you,” meaning that you shall receive what you asked for. Thus the angel endorses your supplication when you pray for your brother unseen and says, “and for you the same.” This demonstrates the merit of the practice.

This, however, applies to someone who has not asked you to pray for him. If a person requests you to pray for him and you comply, he is effectively present, for he hears your words—he is the one who asked you. But when you supplicate for him in his absence, without his informing you and without any request on his part, that is what earns the reward and the virtue. Allah is the One who grants success.