The saying of Allah, exalted be His praise: "And those of your women who commit immorality, call four witnesses from among you against them." [Surah al-Nisa: 15]
And He said: "And those who accuse chaste women and then do not bring four witnesses, flog them with eighty (80) lashes." [Surah al-Nur: 4]
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that Sayyiduna Sa'd (may Allah be pleased with him) said: O Messenger of Allah (peace and blessings be upon him)! If I find a man with my wife, should I then look for four witnesses? The Prophet (peace and blessings be upon him) said: "Yes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20521