Musnad Ahmad 1034:24

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Qatadah narrated to us, from Abu Mijlaz, from Hudhayfah, regarding the one who sits in the middle of the circle: "He is cursed upon the tongue of the Prophet, may Allah bless him and grant him peace, or the tongue of Muhammad, may Allah bless him and grant him peace."