Yahya related to me from Malik that he had heard that Ali ibn AbiTalib used to say that if a man said to his wife, "You are haram forme," it counted as three pronouncements of divorce.Maliksaid, "That is the best of what I have heard on the subject."
USC-MSA web (English) reference: Book 29, Hadith 6
Arabic reference: Book 29, Hadith 1158