It is narrated from Ibn Musayyib that Ali bin Abi Talib (may Allah be pleased with him) was asked about a man who found another man with his wife and killed him, or killed his wife, so he said: "If he does not bring four witnesses, then he should be prepared to be killed himself."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1701