MLA

Chandler, Zachariah, 1813-1879, Hopkins, Moses B, Kellogg, Francis William, 1810-1879, Lincoln, Abraham, 1809-186. [Letter], 1864 May 1 [to] Abraham Lincoln. 1864. https://collections.library.yale.edu/catalog/2026851.

APA, 6th edition

Chandler, Zachariah, 1813-1879, & Hopkins, Moses B, & Kellogg, Francis William, 1810-1879, & Lincoln, Abraham, 1809-186. (1864)[[Letter], 1864 May 1 [to] Abraham Lincoln]. https://collections.library.yale.edu/catalog/2026851.