MLA

Whitman, Walt, 1819-1892. Clemens, Samuel Langhorne. Box 2, folder 80. 1889 May 24. https://collections.library.yale.edu/catalog/2020830.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 80"])(1889 May 24)[Clemens, Samuel Langhorne]. https://collections.library.yale.edu/catalog/2020830.