MLA

Whitman, Walt, 1819-1892. Stead, W. T. (William Thomas) to [Richard?] Le Gallienne. Box 2, folder 104. 1894 Oct 20. https://collections.library.yale.edu/catalog/2053738.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 104"])(1894 Oct 20)[Stead, W. T. (William Thomas) to [Richard?] Le Gallienne]. https://collections.library.yale.edu/catalog/2053738.