Hadith on Supplication: The Virtue of Praying for your brother behind their back

Abu Darda reported: The Messenger of Allah, peace be upon him, said, “There is no Muslim servant who supplicates for his brother behind his back except that the Angel says: and for you the same.”

[Sahih Muslim, Book 35, Number 6588]

عَنْ أَبِي الدَّرْدَاءِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم‏ مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ