Sahih Bukhari Volume 9, Book 92, Hadith Number 409.

Narrated By ‘Asim : I said to Anas, “Did Allah’s Apostle make Medina a sanctuary?” He replied, “Yes, (Medina is a sanctuary from such-and-such place to such-and-such place. It is forbidden to cut its trees, and whoever innovates an heresy in it or commits a sin therein, will incur the curse of Allah, the angels, and all the people.” Then Musa bin Anas told me that Anas added, “…or gives refuge to such an heretic or a sinner…”