Sa'id bin al-Musayyab (may Allah have mercy on him) says that someone from the people of Sham saw another man with his wife and killed them both. Sayyiduna Mu'awiyah (may Allah be pleased with him) was greatly troubled over this judgement, so he wrote about it to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him), asking him to put the question to Sayyiduna Ali (may Allah be pleased with him). Sayyiduna Ali (may Allah be pleased with him) said: "This is something that is not in our land." He said: "Mu'awiyah (may Allah be pleased with him) has asked me." Sayyiduna Ali (may Allah be pleased with him) replied: "I, Abu al-Hasan, say: if he does not bring four witnesses, then retaliation (qisas) is to be taken from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17647