MLA
Soule, Joshua, 1781-1867. Letter from Joshua Soule (Lebanon, Ohio) to Robert Paine - October 28, 1851. October 28, 1851. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/945cc2fs4w-cor.
APA, 6th edition
Soule, Joshua, 1781-1867. (October 28, 1851)[Letter from Joshua Soule (Lebanon, Ohio) to Robert Paine - October 28, 1851]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/945cc2fs4w-cor.
Chicago
Soule, Joshua, 1781-1867, Letter from Joshua Soule (Lebanon, Ohio) to Robert Paine - October 28, 1851, October 28, 1851, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/945cc2fs4w-cor.