Musnad Abu Ya'la al-Mawsili 140:56

Ibrahim ibn Muhammad ibn ‘Ar‘arah narrated to us, Yusuf ibn Ya‘qub narrated to us, Maymun ibn ‘Ajlan narrated to us, from Maymun ibn Siyah, from Anas, from the Prophet (may Allah bless him and grant him peace), who said: "There is no group of people who gather to remember Allah, not desiring by that anything except His Face, but a caller from the heaven calls out to them: 'Rise, you have been forgiven; your evil deeds have been replaced with good deeds.'"