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]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / Q17011
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
Ibn al-Musayyib (may Allah have mercy on him) says that a man in Syria saw another man with his wife and killed him. So Mu'awiyah (may Allah be pleased with him) wrote about it to Abu Musa (may Allah be pleased with him), asking him to put the question to Ali (may Allah be pleased with him). He asked Ali (may Allah be pleased with him), who said: This does not happen in Iraq; tell me (where it happened) and I will give you the answer to it. Then Ali (may Allah be pleased with him) said: I am Abu al-Hasan; if he does not bring four witnesses, then retaliation should be taken from him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17012