Musnad Abu Ya'la al-Mawsili 155:320

Khalaf ibn Hisham narrated to us, Abu al-Ahwas narrated to us, from Abu Ishaq, from al-Agharr Abu Muslim, who said: I bear witness regarding Abu Sa’id and Abu Hurayrah 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 except that the angels surround them, tranquility descends upon them, mercy covers them, and Allah mentions them to those who are with Him."