Musnad Ahmad 32:513

Muhammad ibn Bakr narrated to us, Maymun al-Mar'i informed us, Maymun ibn Siyah narrated to us, from Anas ibn Malik, from the Messenger of Allah, 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.'"