Musannaf Ibn Abi Shayba 27:791

Abu Usama narrated to us, from Ubayd Allah ibn Umar, from Khubayb ibn Abd al-Rahman, from Hafs ibn Asim, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, faith will retreat to Medina just as a snake retreats to its hole."