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

Source: Sahih Muslim 2732

Grade: Sahih (authentic) according to Muslim

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

صحيح مسلم كِتَاب الذِّكْرِ وَالدُّعَاءِ وَالتَّوْبَةِ وَالِاسْتِغْفَارِ بَاب فَضْلِ الدُّعَاءِ لِلْمُسْلِمِينَ بِظَهْرِ الْغَيْبِ 2732