MLA
Wesley, Charles, 1707-1788. Letter from Charles Wesley to Sarah (Gwynne) Wesley, Bristol and Kingswood - May 5, [No Year]. May 5, year unknown. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/071ngf1w2s-cor.
APA, 6th edition
Wesley, Charles, 1707-1788. (May 5, year unknown)[Letter from Charles Wesley to Sarah (Gwynne) Wesley, Bristol and Kingswood - May 5, [No Year]]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/071ngf1w2s-cor.
Chicago
Wesley, Charles, 1707-1788, Letter from Charles Wesley to Sarah (Gwynne) Wesley, Bristol and Kingswood - May 5, [No Year], May 5, year unknown, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/071ngf1w2s-cor.