Musnad Ahmad 30:308

'Affan narrated to us, Wuhayb narrated to us, Suhayl ibn Abi Salih narrated to us, from his father, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "Indeed, Allah has roaming angels, additional [to the scribes], who seek out the gatherings of dhikr (remembrance of Allah)." He then mentioned the rest of the hadith.