First Council of Nicaea, Canon XVI

Neither presbyters, nor deacons, nor any others enrolled among the clergy, who, not having the fear of God before their eyes, nor regarding the ecclesiastical Canon, shall recklessly remove from their own church, ought by any means to be received by another church; but every constraint should be applied to restore them to their own parishes; and, if they will not go, they must be excommunicated.  And if anyone shall dare surreptitiously to carry off and in his own Church ordain a man belonging to another, without the consent of his own proper bishop, from whom although he was enrolled in the clergy list he has seceded, let the ordination be void.

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