Musnad Ahmad 31:874

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 said: I bear witness regarding Abu Hurayrah and Abu Sa‘id that they both bore witness regarding the Prophet, may God bless him and grant him peace, that he said: "No group of people sits remembering God, but the angels surround them, mercy covers them, tranquility descends upon them, and God mentions them to those who are with Him."