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 XXIII

Canon XXIII.

Concerning involuntary homicides, a former decree directs that they be received to full communion after seven years [of penance], according to the prescribed degrees; but this second one, that they fulfil a term of five years.

Notes.

Ancient Epitome of Canon XXIII.

An involuntary homicide shall be subjected to penance for five years.

Van Espen.

Of voluntary and involuntary homicides St. Basil treats at length in his Canonical Epistle ad Amphilochium, can. viii., lvi. and lvii., and fixes the time of penance at twenty years for voluntary and ten years for involuntary homicides.  It is evident that the penance given for this crime varied in different churches, although it is clear from the great length of the penance, how enormous the crime was considered, no light or short penance being sufficient.