Musnad Ahmad 30:2344

Yahya ibn Sa'id al-Umawi narrated to us, he said: 'Ubayd Allah narrated to us, from Khubayb ibn 'Abd al-Rahman, from Hafs ibn 'Asim, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Islam will shrink to Medina just as a snake shrinks to its hole."