MLA
Wesley, Sarah (Gwynne), 1726-1822. Letter from Sarah (Gwynne) Wesley to Sarah Wesley, [London] - August 14, [No Year]. August 14, year unknown. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/013mw6m9jb-cor.
APA, 6th edition
Wesley, Sarah (Gwynne), 1726-1822. (August 14, year unknown)[Letter from Sarah (Gwynne) Wesley to Sarah Wesley, [London] - August 14, [No Year]]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/013mw6m9jb-cor.
Chicago
Wesley, Sarah (Gwynne), 1726-1822, Letter from Sarah (Gwynne) Wesley to Sarah Wesley, [London] - August 14, [No Year], August 14, year unknown, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/013mw6m9jb-cor.