Allah, Exalted be His praise, says: "And those who accuse chaste women and then do not bring four witnesses, flog them eighty lashes and never accept their testimony, and it is they who are the defiantly disobedient." [Surah al-Nur: 4]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / Q17131