Musnad Ahmad 30:2680

Yazid narrated to us, Muhammad ibn ‘Amr informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Al-Madinah: whoever commits an innovation (hadath) in it, or harbors an innovator (muhdith), or pledges allegiance to other than his masters (mawali), upon him is the curse of Allah, the angels, and all of mankind. Allah will not accept from him any obligatory deed (sarf) nor any supererogatory deed (‘adl)."