Muwatta Imam Malik Book of Divorce, Hadith 6

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."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَلِيَّ بْنَ أَبِي طَالِبٍ، كَانَ يَقُولُ فِي الرَّجُلِ يَقُولُ لاِمْرَأَتِهِ أَنْتِ عَلَىَّ حَرَامٌ إِنَّهَا ثَلاَثُ تَطْلِيقَاتٍ . قَالَ مَالِكُ وَذَلِكَ أَحْسَنُ مَا سَمِعْتُ فِي ذَلِكَ .