Allah, the Mighty and Majestic, says: "Then call four witnesses from among you against them." [Surah al-Nisa: 15]
And He said: "Why did they not bring four witnesses for it (the slander)?" [Surah al-Nur: 13]
Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him) said to the Prophet (peace and blessings be upon him): "O Messenger of Allah! If I see some man with my wife, should I leave them be and go looking for four witnesses?" He (peace and blessings be upon him) said: "Yes."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17011