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