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]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / Q20521
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
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that Sa'd bin Ubadah (may Allah be pleased with him) said: If I find a man with my wife, should I then look for four witnesses without saying anything to him? The Messenger of Allah (peace and blessings be upon him) said: "Yes." He said: No, O Messenger of Allah! I would strike his neck first. The Messenger of Allah (peace and blessings be upon him) said: "Listen to what your chief is saying. He is jealous (of his honour), I am more jealous than him, and Allah is more jealous than me."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20522
Sa'id bin al-Musayyib (may Allah have mercy on him) said that Sayyiduna Mu'awiyah (may Allah be pleased with him) wrote to Sayyiduna Abu Musa (may Allah be pleased with him) asking him to ask Sayyiduna Ali (may Allah be pleased with him) about a man who enters his house and finds another man with his wife, and he kills her or kills him. Sayyiduna Abu Musa (may Allah be pleased with him) asked him, and Sayyiduna Ali (may Allah be pleased with him) replied: What reminded you of this? This is something that has not happened in our land. I adjure you (to tell me). Sayyiduna Abu Musa (may Allah be pleased with him) said: Sayyiduna Mu'awiyah (may Allah be pleased with him) wrote to me asking me to ask you about it. He said: I am Abu al-Hasan: if he brings four witnesses (then so be it); otherwise he is to be handed over with his rope (for retaliation). Yahya bin Sa'id (may Allah have mercy on him) said: He is to be killed.
Imam al-Shafi'i (may Allah have mercy on him) said: Three men testified against a man for adultery before Sayyiduna Umar (may Allah be pleased with him), and the fourth did not confirm it, so he flogged the three.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20523
Abu Uthman (may Allah have mercy on him) said: When Abu Bakrah (may Allah be pleased with him) and his two companions testified against al-Mughirah (may Allah be pleased with him), Ziyad came. Hadrat Umar (may Allah be pleased with him) said: Here is a man who, if Allah wills, will testify only to the truth. He said: I saw heavy breathing and an ugly sitting. Hadrat Umar (may Allah be pleased with him) asked: Did you see the kohl stick enter the kohl container? He said: No. So he gave the order concerning them (the three), and they were flogged.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20524