Abu Dharr (RA) reported that Allah's Messenger (ﷺ) said: There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do say: The same be for you too.
Hadith ReferenceSahih Muslim / The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness / Hadith: 2732