Musnad Ahmad 32:1555

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from Asim al-Ahwal, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Al-Madinah is a sanctuary from such-and-such to such-and-such. Whoever commits an innovation (hadath) therein, or shelters an innovator (muhdith), upon him is the curse of Allah, the angels, and all of mankind. Its trees shall not be cut." He said: And al-Hasan said: "Except for the fodder of a camel."