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 V

Canon V.

If a catechumen coming into the Church have taken his place in the order of catechumens, and fall into sin, let him, if a kneeler, become a hearer and sin no more.  But should he again sin while a hearer, let him be cast out.

Notes.

Ancient Epitome of Canon V.

If a catechumen falls into a fault and if while a kneeler he sins no more, let him be among the hearers; but should he sin while among the hearers, let him be cast out altogether.

Zonaras.

There are two sorts of catechumens.  For some have only just come in and these, as still imperfect, go out immediately after the reading of the scriptures and of the Gospels.  But there are others who have been for some time in preparation and have attained some perfection; these wait after the Gospel for the prayers for the catechumens, and when they hear the words “Catechumens, bow down your heads to the Lord,” they kneel down.  These, as being more perfect, having tasted the good words of God, if they fall, are removed from their position; and are placed with the “hearers” but if any happen to sin while “hearers” they are cast out of the Church altogether.