Second Council of Nicaea, Canon III

That it does not pertain to princes to choose a Bishop.

Let every election of a bishop, presbyter, or deacon, made by princes stand null, according to the canon which says:  If any bishop making use of the secular powers shall by their means obtain jurisdiction over any church, he shall be deposed, and also excommunicated, together with all who remain in communion with him.  For he who is raised to the episcopate must be chosen by bishops, as was decreed by the holy fathers of Nice in the canon which says:  It is most fitting that a bishop be ordained by all the bishops in the province; but if this is difficult to arrange, either on account of urgent necessity, or because of the length of the journey, three bishops at least having met together and given their votes, those also who are absent having signified their assent by letters, the ordination shall take place.  The confirmation of what is thus done, shall in each province be given by the metropolitan thereof.

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