Council of Chalcedon, Canon XVI

It is not lawful for a virgin who has dedicated herself to the Lord God, nor for monks, to marry; and if they are found to have done this, let them be excommunicated.  But we decree that in every place the bishop shall have the power of indulgence towards them.

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