Assalaamu alaykum! I want to know the authenticity of the hadith given by At-Tirmithi and reported by Sufyaan Ath-Thawry "That a man who wants to keep a woman with whom he made Halala has to remake a new contract because the intention of Halala made this contract void". Is it possible to have the graduate of this hadith, and if it is weak, does that mean that this man is fordbidden for this woman for his entire life?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger. 

At-Tirmithi cited this report on the authority of Sufyaan Ath-Thawri in his book Sunan At-Tirmithi after citing the hadeeth about the Muhallil (a man who marries an irrevocably divorced woman with major separation just to make her lawful for her ex-husband again). This statement was also cited by Al-Marwazi, as he said, “Sufyaan was once asked about a man who marries a woman merely to make her lawful to her ex-husband and then changes his mind and decides to retain his marriage with her. He replied, ‘I do not like except that he terminates this marriage and remarries her anew.’” [Masaa'il Imaam Ahmad Ibn Hanbal and Is-haaq ibn Raahawayh] Ahmad declared the report good.

We could not find the chain of narration of the report in reference narrated on the authority of Sufyaan Ath-Thawri; however, scholars attribute this statement to him as they do with other scholars. The authenticity of this report is irrelevant in this regard because the meaning of his statement conforms to the view of the majority of scholars declaring Tahleel marriage invalid and underlining that the woman remains unlawful for her ex-husband unless she marries another man and establishes conjugal relations with her new husband in a valid marriage.

We are not sure to whom you are referring at the end of the question, the ex-husband or the Muhallil. If you are referring to the Muhallil, then some scholars held that he is unlawful for her his whole life. This is the view of the Maalikis, contrary to the opinion of the Shaafi‘i and Hanbalis, who held that he is not unlawful for her his entire life.

For more benefit, please refer to fatwa 329044.

Allaah knows best.