MLA

Paul, the Deacon, approximately 720-799?. Historia gentis Longobardorum, etc. [between 1450 and 1500]. https://collections.library.yale.edu/catalog/11403269.

APA, 6th edition

Paul, the Deacon, approximately 720-799?. ([between 1450 and 1500])[Historia gentis Longobardorum, etc]. https://collections.library.yale.edu/catalog/11403269.