Musnad Abu Ya'la al-Mawsili 42:279

Zuhayr narrated to us, Abu al-Walid narrated to us, Shu'bah narrated to us, Abu Ishaq informed us, he said: I heard al-Agharr Abu Muslim saying: I bear witness regarding Abu Sa'id al-Khudri 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 mercy covers them, the angels surround them, tranquility descends upon them, and Allah mentions them to those who are with Him."