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