“Whoever seeks forgiveness for the believing men and women Allah will record one good deed corresponding to every believing man and woman”
Tabaraani. According to Albani it is Hassan.
ما من عبد مسلم يدعو لأخيه بظهر الغيب إلا قال المَلَكُ ولك بمثل
“No Muslim servant (of Allah) prays for his brother behind his back except that an angel says, “And for you the same.”
Abu ‘Abdullah bin al-Khatib narrated:
“Abu Hamdun (one of the Tabi’in) had a scroll on which was written the names of three hundred of his close friends, and he used to supplicate for them – one by one – every single night.”
[‘Sifat as-Safwah’; 1/492]