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]

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.]