Al-A'mash (may Allah have mercy on him) said: There was an old man in Kufah who used to say: I heard Ali bin Abi Talib (may Allah be pleased with him) say: "When a man divorces his wife three times in a single sitting, it is reduced to one." At that time people were flocking to him, coming to him and listening to him. He said: So I went to him and knocked on his door, and an old man came out to me. I said to him: "What did you hear Ali bin Abi Talib (may Allah be pleased with him) say about one who divorces his wife three times in a single sitting?" He said: "I heard Ali bin Abi Talib (may Allah be pleased with him) say: When a man divorces his wife three times in a single sitting, it is reduced to one." I said to him: "Where did you hear this from Ali (may Allah be pleased with him)?" He said: "I will bring out a book for you." He brought it out, and in it was written: 'Bismillahir-Rahmanir-Rahim' (In the name of Allah, the Most Gracious, the Most Merciful). This is what I heard Ali bin Abi Talib (may Allah be pleased with him) say: When a man divorces his wife three times in a single sitting, she is separated from him and is not lawful for him until she marries another husband. I said: "Woe to you! This is other than what you say." He said: "The correct one is this, but these people pressed me to say that."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14988