First Council of Nicaea, Canon II

Forasmuch as, either from necessity, or through the urgency of individuals, many things have been done contrary to the Ecclesiastical canon, so that men just converted from heathenism to the faith, and who have been instructed but a little while, are straightway brought to the spiritual laver, and as soon as they have been baptized, are advanced to the episcopate or the presbyterate, it has seemed right to us that for the time to come no such thing shall be done.  For to the catechumen himself there is need of time and of a longer trial after baptism.  For the apostolical saying is clear, “Not a novice; lest, being lifted up with pride, he fall into condemnation and the snare of the devil.”  But if, as time goes on, any sensual sin should be found out about the person, and he should be convicted by two or three witnesses, let him cease from the clerical office.  And whoso shall transgress these [enactments] will imperil his own clerical position, as a person who presumes to disobey the great Synod.

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