Surah An-Nur, Verse 13

لَّوۡلَا جَآءُو عَلَيۡهِ بِأَرۡبَعَةِ شُهَدَآءَۚ فَإِذۡ لَمۡ يَأۡتُواْ بِٱلشُّهَدَآءِ فَأُوْلَٰٓئِكَ عِندَ ٱللَّهِ هُمُ ٱلۡكَٰذِبُونَ١٣

lawlā jāū ʿalayhi bi-arbaʿati shuhadāa fa-idh lam yatū bil-shuhadāi fa-ulāika ʿinda l-lahi humu l-kādhibūna

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.