Ibn Abbas (may Allah be pleased with them both) said: "Why did they not bring four witnesses to it?" — had they done so, they and those who testified would all have been deemed liars — "And since they did not bring the witnesses, it is they who are the liars in the sight of Allah" [Surah An-Nur: 13], meaning sheer lying.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19494
Sa'id bin Jubayr said concerning this verse: "Why did they not bring to it" — meaning to the accusation — "four witnesses? And since they did not bring the witnesses, it is they who are the liars in the sight of Allah" [Surah An-Nur: 13], meaning those who accused Sayyidah Aishah.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19495
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