Abu Sakhr relates: "Why did they not bring four witnesses to it? And since they did not bring the witnesses, it is they who are the liars in the sight of Allah": Whoever accuses a Muslim and then cannot bring four witnesses is a qadhif (false accuser), and the hadd punishment for false accusation (qadhf) becomes binding upon him.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19496