Musnad Ahmad 30:1584

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, Suhayl ibn Abi Salih narrated to us, from his father, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah, the Almighty and Majestic, has angels who travel about, seeking out the gatherings of dhikr (remembrance of Allah)." Then he mentioned the rest of it.