Second Council of Constantinople, Capitulum VIII

If anyone uses the expression “of two natures,” confessing that a union was made of the Godhead and of the humanity, or the expression “the one nature made flesh of God the Word,” and shall not so understand those expressions as the holy Fathers have taught, to wit:  that of the divine and human nature there was made an hypostatic union, whereof is one Christ; but from these expressions shall try to introduce one nature or substance [made by a mixture] of the Godhead and manhood of Christ; let him be anathema.  For in teaching that the only-begotten Word was united hypostatically [to humanity] we do not mean to say that there was made a mutual confusion of natures, but rather each [nature] remaining what it was, we understand that the Word was united to the flesh.  Wherefore there is one Christ, both God and man, consubstantial with the Father as touching his Godhead, and consubstantial with us as touching his manhood.  Therefore they are equally condemned and anathematized by the Church of God, who divide or part the mystery of the divine dispensation of Christ, or who introduce confusion into that mystery.

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