Second Council of Constantinople, Capitulum V

If anyone understands the expression “one only Person of our Lord Jesus Christ” in this sense, that it is the union of many hypostases, and if he attempts thus to introduce into the mystery of Christ two hypostases, or two Persons, and, after having introduced two persons, speaks of one Person only out of dignity, honour or worship, as both Theodorus and Nestorius insanely have written; if anyone shall calumniate the holy Council of Chalcedon, pretending that it made use of this expression [one hypostasis] in this impious sense, and if he will not recognize rather that the Word of God is united with the flesh hypostatically, and that therefore there is but one hypostasis or one only Person, and that the holy Council of Chalcedon has professed in this sense the one Person of our Lord Jesus Christ:  let him be anathema.  For since one of the Holy Trinity has been made man, viz.:  God the Word, the Holy Trinity has not been increased by the addition of another person or hypostasis.

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