Mattityahu 5:31-32

It was also said, Whoever divorces his wife, let him give to her a “get,” a SEFER KERITUT (bill of divorce). But I say to you that everyone divorcing his wife, except for the DVAR (indecent thing) of zenut (fornication), makes her become a noefet (adulteress), and whoever marries a gerusha (divorcee) commits niuf (adultery).