MLA

Whitman, Walt, 1819-1892. Stewart, William. Box 1, folder 32. 1865 Jul 17. https://collections.library.yale.edu/catalog/2053666.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 32"])(1865 Jul 17)[Stewart, William]. https://collections.library.yale.edu/catalog/2053666.