Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon XXXVIII

Canon XXXVIII.  (Greek xli.)

That clerics or those who are continent shall not visit virgins or widows.

Neither clerics nor those who profess continence should enter the houses of widows or virgins without the bidding or consent of the bishops or presbyters:  and then let them not go alone, but with some other of the clergy, or with those assigned by the bishop or presbyter for this purpose; not even bishops and presbyters shall go alone to women of this sort, except some of the clergy are present or some other grave Christian men.

Notes.

Ancient Epitome of Canon XXXVIII.

Clerics and those who are continent shall not go to widows or virgins, unless at the bidding of the bishop and presbyter:  and even then not alone, but with those with whom presbyters and deacons visit them.

This canon is canon xxiv. of the Synod of Hippo, a.d. 393.