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." (IT)
USC-MSA web (English) reference: Book 29, Hadith 6 (IT)
Arabic reference (IT): Book 29, Hadith 1158 (IT)