Hadith: 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

عن أم الدرداء رضي الله عنها مرفوعاً: « دعوة المرء المسلم لأخيه بظَهْرِ الغيب مستجابة، عند رأسه مَلَك مُوَكَّلٌ كلما دعا لأخيه بخير قال الملك المُوَكَّلُ به: آمين، ولك بمِثْلٍ».
[صحيح] - [رواه مسلم]

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.'"
[Authentic hadith] - [Narrated by Muslim]

Explanation

Allah accepts the invocation that a Muslim makes for his brother, who is absent and does not know about it. So when he supplicates, an angel stands at his head and says, “Amen,” that may you have the same good that you have invoked for your brother.