It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: When a group gathers together to remember Allah, and their only purpose is to seek Allah’s pleasure, then a caller calls out from the heavens: You are standing in such a state that your sins have been forgiven, and I have replaced your sins with good deeds.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12453
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد حسن