An-Nur 24:13

Lawla jaoo AAalayhibi-arbaAAati shuhadaa fa-ith lam ya/too bishshuhada-ifaola-ika AAinda Allahi humu alkathiboon

Why did they [who slandered] not produce for it four witnesses? And when they do not produce the witnesses, then it is they, in the sight of Allāh, who are the liars.

Why did they (the accusers) not bring four witnesses to prove this? So, as they did not bring the witnesses, they are the liars in the sight of Allah.