MLA
Wesley, Sarah, 1759-1828. Letter from Sarah Wesley to Samuel Wesley, "A letter to be delivered after my death" - September 2, 1807. September 2, 1807. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/600ns1rntx-cor.
APA, 6th edition
Wesley, Sarah, 1759-1828. (September 2, 1807)[Letter from Sarah Wesley to Samuel Wesley, "A letter to be delivered after my death" - September 2, 1807]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/600ns1rntx-cor.
Chicago
Wesley, Sarah, 1759-1828, Letter from Sarah Wesley to Samuel Wesley, "A letter to be delivered after my death" - September 2, 1807, September 2, 1807, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/600ns1rntx-cor.