Musnad Abu Ya'la al-Mawsili 42:310

Zuhayr narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Abu Ishaq, from al-Agharr Abu Muslim, who said: I bear witness regarding Abu Hurayrah and Abu Sa'id that they both bore witness regarding the Messenger of Allah, may Allah bless him and grant him peace, that he said: "No people sit remembering Allah, the Mighty and Majestic, except that the angels surround them, mercy covers them, tranquility descends upon them, and Allah mentions them to those who are with Him."