Maliks Muwatta Book 29, Hadith Number 6.
Section : The ‘Irrevocable’ Divorce.
Yahya related to me from Malik that he had heard that Ali ibn Abi Talib used to say that if a man said to his wife, “You are haram for me,” it counted as three pronouncements of divorce.
Malik said, “That is the best of what I have heard on the subject.”