Umm Ad-Dardā’ (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: “The supplication of a Muslim for a fellow Muslim in his absence is answered. At his head, there is an entrusted angel. Whenever one invokes good for his brother, the entrusted angel says: 'Amen, and likewise to you.'" [Muslim].
The supplication of a Muslim for his brother in his absence
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