Musnad Ahmad 31:297

Waki’ narrated to us, from Isra’il, 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 to me regarding the Messenger of Allah (peace and blessings of Allah be upon him) that he said—and I bear witness against them both—: "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."