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

عَن أُمِّ الدَّرْدَاءِ وَ أَبِي الدَّرداءِ رَضيَ اللهُ عنهما أَنَّ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَانَ يَقُولُ:
«دَعْوَةُ الْمَرْءِ الْمُسْلِمِ لِأَخِيهِ بِظَهْرِ الْغَيْبِ مُسْتَجَابَةٌ، عِنْدَ رَأْسِهِ مَلَكٌ مُوَكَّلٌ كُلَّمَا دَعَا لِأَخِيهِ بِخَيْرٍ، قَالَ الْمَلَكُ الْمُوَكَّلُ بِهِ: آمِينَ وَلَكَ بِمِثْلٍ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 2733]

’Umm ad-Dardā’ and Abu ad-Dardā’ (may Allah be pleased with them) reported that the Prophet (may Allah's peace and blessings be upon him) used to say:
"The supplication of a Muslim for a fellow Muslim in his absence is answered. At his head, there is an entrusted angel. Whenever he invokes good for his brother, the entrusted angel says: Amen, and likewise to you."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 2733]

Explanation

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.

Benefits from the Hadith

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.