Quran 24:4

وَٱلَّذِينَ يَرۡمُونَ ٱلۡمُحۡصَنَـٰتِ ثُمَّ لَمۡ يَأۡتُواْ بِأَرۡبَعَةِ شُہَدَآءَ فَٱجۡلِدُوهُمۡ ثَمَـٰنِينَ جَلۡدَةً۬ وَلَا تَقۡبَلُواْ لَهُمۡ شَہَـٰدَةً أَبَدً۬ا‌ۚ وَأُوْلَـٰٓٮِٕكَ هُمُ ٱلۡفَـٰسِقُونَ ٤

Those who accuse married women of adultery, then fail to produce four witnesses, you shall whip them eighty lashes, and do not accept any testimony from them; they are wicked.