Is it ensure, insure, or assure?
1580s, "act of referring," from refer + -ance, or else from French référence, from Medieval Latin *referentia, from Latin referentem (nominative referens), present participle of referre (see refer). Meaning "direction to a book or passage" is recorded from 1610s. Meaning "testimonial" is from 1895. Reference book dates from 1808. Phrase in reference to is attested from 1590s.
1620s, "to assign;" as "to provide with a reference," 1837 (implied in referenced), from reference (n.). Related: Referencing.