The Third Ecumenical Council:  The Council of Ephesus 7Anathematism III.

III. If anyone shall after the [hypostatic] union divide the hypostases in the one Christ, joining them by that connexion alone, which happens according to worthiness, or even authority and power, and not rather by a coming together (sunodo), which is made by natural union (enosin phusiken): let him be anathema.