Musnad Abu Ya'la al-Mawsili 155:317

Abu Bakr ibn Abi Shaybah narrated to us, Yahya ibn Adam narrated to us, from Ammar ibn Ruzayq, from Abu Ishaq, from al-Agharr Abu Muslim, from Abu Hurayrah and Abu Sa’id, they both said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "No group of Muslims sits remembering Allah, but the angels surround them, mercy covers them, and tranquility descends upon them."