Council of Chalcedon, Canon XVIII

The crime of conspiracy or banding together is utterly prohibited even by the secular law, and much more ought it to be forbidden in the Church of God.  Therefore, if any, whether clergymen or monks, should be detected in conspiring or banding together, or hatching plots against their bishops or fellow-clergy, they shall by all means be deposed from their own rank.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.