Nicæa I: canon XVI

Any presbyters or deacons or in general anyone enrolled in any rank of the clergy who depart from their church recklessly and without the fear of God before their eyes or in ignorance of the church's canon, ought not by any means to be received in another church, but all pressure must be applied to them to induce them to return to their own dioceses, or if they remain it is right that they should be excommunicated. But if anyone dares to steal away one who belongs to another and to ordain him in his church without the consent of the other's own bishop among whose clergy he was enrolled before he departed, the ordination is to be null.