Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "Madinah, from here to here, is a sacred and safe place. Its trees are not to be cut, nor is any innovation to be introduced in it. Whoever introduces an innovation in it, upon him is the curse of Allah, the angels and all the people; neither his obligatory nor his voluntary deeds will be accepted from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9959