He was asked about a man who divorces his wife, then has intercourse with her, without calling witnesses to the divorce or to taking her back. He replied, "You divorced contrary to the sunnah, and you took her back contrary to the sunnah. Call witnesses to her divorce and to her return, and do not do this again."
سئل عن الرجل يطلق امرأته، ثم يقع بها، ولم يشهد على طلاقها، ولا على رجعتها، فقال: طلقت لغير سنة، وراجعت لغير سنة، و أشهد على طلاقها وعلى رجعتها، ولا تعد.
Abu Dawud (2186) and Ibn Majah (2025) both narrated it from Bashir bin Hilal Sawwaf, who said, Jafar bin Sulayman Dubai narrated to us, from Yazid Rishk, from Mutarrif bin Abdullah bin Shikhir, that Imran bin Husayn, then he mentioned it. Its chain is hasan because of Jafar bin Sulayman Dubai, for his hadith is hasan. The discussion of it is elaborated in the Book of Divorce.The command to bring witnesses here is for recommendation, like His saying, Most High, "and call witnesses when you make a contract of trade" [Baqarah 2:282]. Calling witnesses in a trade contract is not obligatory, so likewise calling witnesses at reconciliation or at the resolve to separate is not obligatory.And His saying, "And whoever relies upon Allah, then He is sufficient for him" [Talaq 65:3], means that Allah protects him and suffices him, even if people become his enemies and oppose him. This has indeed come in the hadith.