Malik related to me from Yahya ibn Said from Said ibn al-Musayyabthat a Syrian man called Ibn Khaybari found a man with his wife andkilled him, or killed them both. Muawiya ibn Abi Sufyan found itdifficult to make a decision and he wrote to Abu Musa al-Ashari to askAli ibn Abi Talib for him about that. So Abu Musa asked Ali ibn AbiTalib and AIi said to him, "Is this thing in my land? I adjure you,you must tell me." Abu Musa explained to him how Muawiya ibn AbiSufyan had written him to ask Ali about it. Ali said, "I am Abu Hasan.If he does not bring four witnesses, then let him be completely handedover," (to the relatives of the murdered man).
USC-MSA web (English) reference: Book 36, Hadith 18
Arabic reference: Book 36, Hadith 1422