Sahih Muslim Book 7, Hadith Number 3166.
Chapter : Eminence of Medina, Allah’s Apostle (may peace be upon him) showered blessings upon it. Its sacredness and unlawfulness of hunting and lopping of trees and demarcation its precincts.
Abu Huraira (Allah be pleased with him) reported Allah’s Apostle (may peace be upon him) as saying: Medina is a sacred territory, so he who made any innovation in it or gave protection to an innovator, there is upon him the curse of Allah, that of the angels and that of all the people. There would not be accepted on the Day of Resurrection either obligatory acts or supererogatory acts from him.