Your paper is in literary studies or another field concerned with close analysis of text.
If differences in translations are relevant to your paper.
If editions differ in wording, line numbering, etc.
Ask your professor which citation form you should use if not sure.
Example of citation:
4. Augustine De civitate Dei 20.2
5. Abelard Epistle 17 to Heloise (Migne PL 180.375c-378a).
(Abelard Epistle 17 to Heloise, Migne PL 180.375c-378a)
(Augustine De civitate Dei 20.2)