Hadith 1494
Abud-Darda' (May Allah be pleased with him) reported:
وعن أبي الدرداء رضي الله عنه أنه سمع رسول الله صلى الله عليه وسلم يقول: "ما من عبد مسلم يدعو لأخيه بظهر الغيب إلا قال الملك ولك بمثل" ((رواه مسلم)).
I heard the Messenger of Allah (ﷺ) saying, "Whenever a Muslim supplicates for his (Muslim) brother in his absence, the angels say: 'May the same be for you too'." [Muslim].
In plain words
When you pray for a fellow Muslim who is not present, an angel asks that the same good come to you. Quiet kindness for others returns to the one who offers it.
Reference : Riyad as-Salihin 1494 In-book reference : Book 16, Hadith 30