Musnad Abu Ya'la al-Mawsili 155:28

Abdullah ibn Umar ibn Aban narrated to us, Ishaq ibn Sulayman narrated to us, from Mu'awiyah ibn Yahya, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say—and he mentioned Medina—: "There will be conquests, and there will be people who will wander away with their tribes, while Medina is better for them, if only they knew."