MLA

Whitman, Walt, 1819-1892. "Ell" to unidentified. Box 1, folder 40. n.d.. https://collections.library.yale.edu/catalog/2053674.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 40"])(n.d.)["Ell" to unidentified]. https://collections.library.yale.edu/catalog/2053674.