MLA

Casus Breves Decretalium Gregorii IX (Fragment). 0AD. https://collections.library.yale.edu/catalog/2003680.

APA, 6th edition

Casus Breves Decretalium Gregorii IX (fragment). (0 C.E.). [Commentaries, Manuscripts, Medieval Italy 13th century, Medieval and Renaissance Manuscript Fragments in Beinecke Library, Medieval and Renaissance Manuscripts in Beinecke Library]. https://collections.library.yale.edu/catalog/2003680.