MLA

Whitman, Walt, 1819-1892. Rossetti, William Michael to The Sun. Box 2, folder 103. [1868?] Apr 23. https://collections.library.yale.edu/catalog/2053737.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 103"])([1868?] Apr 23)[Rossetti, William Michael to The Sun]. https://collections.library.yale.edu/catalog/2053737.