The supplication of a Muslim for a fellow Muslim in his absence is answered

The Prophet (may Allah's peace and blessings be upon him) informed that a Muslim's supplication for his fellow Muslim in his absence is answered because it demonstrates greater sincerity. At the head of the supplicator is an appointed angel who says whenever he invokes good for his brother: Amen, and may you receive the same.

1. Encouraging believers to show kindness to one another, even with supplication.
2. Supplicating for others in their absence is an indication of sincere faith and true brotherhood.
3. Restricting supplication for others to their absence, because this demonstrates greater sincerity and presence of heart.
4. One of the reasons for supplications being answered is when a Muslim prays for his fellow Muslim in his absence.
5. An-Nawawi said: The merit of supplicating for a fellow Muslim in his absence—even if one prays for a group of Muslims, this merit is attained. And if one supplicates for the entire Muslim community, it is evident that this merit is also attained. Some of the righteous predecessors, when they wished to supplicate for themselves, would supplicate for their Muslim brother with the same supplication, because it is answered, and they receive the same.
6. Clarifying some of the tasks of angels—among them are those whom Allah has entrusted with this specific task.

صحيح مسلم (4/ 2094) (2733). بهجة الناظرين شرح رياض الصالحين، لسليم الهلالي (2/ 585). نزهة المتقين شرح رياض الصالحين، لمجموعة من الباحثين (2/ 1018). شرح رياض الصالحين، لابن عثيمين (6/ 48). كنوز رياض الصالحين، لمجموعة من الباحثين (17/ 510).