pièce justificative
/piːˌɛs ˌʒuːstɪfɪkəˈtiːv/noun
A document serving as proof of an allegation; a justification of an assertion.
Origin
Late 18th century; earliest use found in Hannah More (1745–1833), writer and philanthropist. From French pièce justificative from pièce + justificative, feminine of justificatif.