Musnad Ahmad 30:728

Hammad ibn Usamah narrated to us, Ubaydullah narrated to us, from Khubayb ibn Abd al-Rahman, from Hafs ibn Asim, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, faith will retreat to Medina, just as a snake retreats to its hole."