MLA

Whitman, Walt, 1819-1892. Murray, Samuel. Box 11, folder 231. 1891. https://collections.library.yale.edu/catalog/2053856.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 11, folder 231"])(1891)[Murray, Samuel]. https://collections.library.yale.edu/catalog/2053856.