MLA
Wesley, Sarah, 1759-1828. Letter from Sarah Wesley to Sarah (Gwynne) Wesley, [London] - September 7, [No Year]. September 7, year unknown. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/064tht771k-cor.
APA, 6th edition
Wesley, Sarah, 1759-1828. (September 7, year unknown)[Letter from Sarah Wesley to Sarah (Gwynne) Wesley, [London] - September 7, [No Year]]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/064tht771k-cor.
Chicago
Wesley, Sarah, 1759-1828, Letter from Sarah Wesley to Sarah (Gwynne) Wesley, [London] - September 7, [No Year], September 7, year unknown, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/064tht771k-cor.