لَوْلَا جَاۤءُوْ عَلَيْهِ بِاَرْبَعَةِ شُهَدَاۤءَۚ فَاِذْ لَمْ يَأْتُوْا بِالشُّهَدَاۤءِ فَاُولٰۤىِٕكَ عِنْدَ اللّٰهِ هُمُ الْكٰذِبُوْنَ ( النور: ١٣ )
Law laa jaaa'oo 'alaihi bi-arba'ati shuhadaaa'; fa iz lam yaatoo bishshuhadaaa'i fa ulaaa 'ika 'indal laahi humul kaaziboon (an-Nūr 24:13)
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 Allah, who are the liars. (An-Nur  : 13)
1 Mufti Taqi Usmani
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.