Musnad Abu Ya'la al-Mawsili 155:319

Muhammad ibn Bashar narrated to us, Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Abu Ishaq narrating from al-Agharr Abu Muslim, that he bore 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 except that the angels surround them, mercy covers them, tranquility descends upon them, and Allah mentions them to those who are with Him."