MLA
Wesley, John, 1703-1791. Letter from John Wesley to "Sally" Sarah (Mallet) Boyce, Canterbury - December 15, 1789. December 15, 1789. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/0580k6dk0t-cor.
APA, 6th edition
Wesley, John, 1703-1791. (December 15, 1789)[Letter from John Wesley to "Sally" Sarah (Mallet) Boyce, Canterbury - December 15, 1789]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/0580k6dk0t-cor.
Chicago
Wesley, John, 1703-1791, Letter from John Wesley to "Sally" Sarah (Mallet) Boyce, Canterbury - December 15, 1789, December 15, 1789, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/0580k6dk0t-cor.