Asim (may Allah have mercy on him) says: I asked Hazrat Anas (may Allah be pleased with him), "Did the Prophet (peace and blessings of Allah be upon him) declare Madinah Munawwarah as a sanctuary (Haram)?" He replied, "Yes! It is a sanctuary. Allah and His Messenger have declared it a sanctuary. Even its grass cannot be cut. Whoever does so, upon him is the curse of Allah, the angels, and all people."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 13063