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 XL

p. 152 Canon XL.

Bishops called to a synod must not be guilty of contempt, but must attend, and either teach, or be taught, for the reformation of the Church and of others.  And if such an one shall be guilty of contempt, he will condemn himself, unless he be detained by ill health.

Notes.

Ancient Epitome of Canon XL.

Whoso summoned to a synod shall spurn the invitation, unless hindered by the force of circumstances, shall not be free from blame.

Hefele.

By ἀνωμαλία, illness is commonly understood, and Dionysius Exiguus and Isidore translated it, the former ægritudinem, and the latter infirmitatem.  But Balsamon justly remarks that the term has a wider meaning, and, besides cases of illness includes other unavoidable hinderances or obstacles.

This Canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. XVIII., c. v.