MLA

Blair, Montgomery, 1813-1883, Lincoln, Abraham, 1809-1865, Miller, William Winlock, 1822-187, United States. President (1861-1865 : Lincoln. [Letter], 1864 Feb 23 [to] Postmaster General. 1864. https://collections.library.yale.edu/catalog/2026850.

APA, 6th edition

Blair, Montgomery, 1813-1883, & Lincoln, Abraham, 1809-1865, & Miller, William Winlock, 1822-187, & United States. President (1861-1865 : Lincoln. (1864)[[Letter], 1864 Feb 23 [to] Postmaster General]. https://collections.library.yale.edu/catalog/2026850.