Sahih Muslim Book 7, Hadith Number 3159.

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.

‘Asim reported: I asked Anas b. Malik whether Allah’s Messenger (may peace be upon him) had declared Medina as sacred. He said: Yes. (the area) between so and so. He who made any innovation in it, and further said to me: It is something serious to make any innovation in it (and he who does it) there is upon him the curse of Allah, and that of the angels and of all the people, Allah will not accept from him on the Day of Resurrection either obligatory acts or the supererogatory acts. Ibn Anas said: Or he accommodates an innovator.