MLA

Whitman, Walt, 1819-1892. Burroughs, John. Box 1, folder 3. 1867 Sep 21. https://collections.library.yale.edu/catalog/2053637.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 3"])(1867 Sep 21)[Burroughs, John]. https://collections.library.yale.edu/catalog/2053637.