Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Councils of Ancyra, Gangra, Neocæsarea, Antioch and Laodicea, which Canons were Accepted and Received by the Ecumenical Synods.: Canon III

Canon III.

He who has been recently baptized ought not to be promoted to the sacerdotal order.

Notes.

Ancient Epitome of Canon III.

A neophite is not ordainable.

This rule is laid down in the Second Nicene canon.  Balsamon also compares Apostolic Canon lxxx.

Balsamon.

Notwithstanding this provision, that great light, Nectarius, just separated from the flock of the catechumens, when he had washed away the sins of his life in the divine font, now pure himself, he put on the most pure dignity of the episcopate, and at the same time became bishop of the Imperial City, and president of the Second Holy Ecumenical Synod.