Council of Chalcedon, Canon XIV

Since in certain provinces it is permitted to the readers and singers to marry, the holy Synod has decreed that it shall not be lawful for any of them to take a wife that is heterodox.  But those who have already begotten children of such a marriage, if they have already had their children baptized among the heretics, must bring them into the communion of the Catholic Church; but if they have not had them baptized, they may not hereafter baptize them among heretics, nor give them in marriage to a heretic, or a Jew, or a heathen, unless the person marrying the orthodox child shall promise to come over to the orthodox faith.  And if any one shall transgress this decree of the holy synod, let him be subjected to canonical censure.

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