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 LVI

Canon LVI.

Presbyters may not enter and take their seats in the bema before the entrance of the Bishop:  but they must enter with the Bishop, unless he be at home sick, or absent.

Notes.

Ancient Epitome of Canon LVI.

A presbyter shall not enter the bema before the bishop, nor sit down.

It is difficult to translate this canon without giving a false idea of its meaning.  It does not determine the order of dignity in an ecclesiastical procession, but something entirely different, viz., it provides that when the bishop enters the sanctuary he should not be alone and walk into a place already occupied, but that he should have with him, as a guard p. 158 of honour, the clergy.  Whether these should walk before or after him would be a mere matter of local custom, the rule juniores priores did not universally prevail.

This canon is found in the Corpus Juris Canonici, Gratian’s Decretum, Pars I., Dist. XCV., can. viij.