Musnad Ahmad 31:473

'Abd al-Rahman narrated to us, from Sufyan, 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, peace and blessings of Allah be upon him, that he said: "No people sit remembering Allah, except that the angels surround them, mercy covers them, and Allah mentions them to those who are with Him."