Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
Appendix containing Canons and Rulings not having Conciliar Origin but Approved by Name in Canon II. of the Synod in Trullo.: From the Letter of Basil the Great to the Nicopolitans.

p. 611 From the Letter of Basil the Great to the Nicopolitans.

There is also in Tilius and Bishop Beveridge here 599 inserted an epistle of St. Basil the Great to the Nicopolitans, comforting them under the loss of their church or oratory, and telling them, that they ought not to be concerned that they worship God in the open air, for that the eleven Apostles worshipped God in an upper room, where they were cooped up, while they that crucified Jesus performed their worship in a most famous Temple.

Footnotes

I.e., at the end, after the Epistle of Gennadius.