Narrated Mutarrif ibn Abdullah: Imran ibn Husayn was asked about a person who divorces his wife, and then has intercourse with her, but he does not call any witness to her divorce nor to her restoration. He said: You divorced against the sunnah and took her back against the sunnah. Call someone to bear witness to her divorce, and to her return in marriage, and do not repeat it.
Sunan Abi Dawud 2186
Chapters
1 Purification (Kitab Al-Taharah) 2 Prayer (Kitab Al-Salat) 3 The Book Of The Prayer For Rain (Kitab al-Istisqa') 4 Prayer (Kitab Al-Salat): Detailed Rules of Law about the Prayer during Journey 5 Prayer (Kitab Al-Salat): Voluntary Prayers 6 Prayer (Kitab Al-Salat): Detailed Injunctions about Ramadan 7 Prayer (Kitab Al-Salat): Prostration while reciting the Qur'an 8 Prayer (Kitab Al-Salat): Detailed Injunctions about Witr 9 Zakat (Kitab Al-Zakat) 10 The Book of Lost and Found Items 11 The Rites of Hajj (Kitab Al-Manasik Wa'l-Hajj) 12 Marriage (Kitab Al-Nikah) 13 Divorce (Kitab Al-Talaq) 14 Fasting (Kitab Al-Siyam) 15 Jihad (Kitab Al-Jihad) 16 Sacrifice (Kitab Al-Dahaya) 17 Game (Kitab Al-Said) 18 Wills (Kitab Al-Wasaya) 19 Shares of Inheritance (Kitab Al-Fara'id) 20 Tribute, Spoils, and Rulership (Kitab Al-Kharaj, Wal-Fai' Wal-Imarah) 21 Funerals (Kitab Al-Jana'iz) 22 Oaths and Vows (Kitab Al-Aiman Wa Al-Nudhur) 23 Commercial Transactions (Kitab Al-Buyu) 24 Wages (Kitab Al-Ijarah) 25 The Office of the Judge (Kitab Al-Aqdiyah) 26 Knowledge (Kitab Al-Ilm) 27 Drinks (Kitab Al-Ashribah) 28 Foods (Kitab Al-At'imah) 29 Medicine (Kitab Al-Tibb) 30 Divination and Omens (Kitab Al-Kahanah Wa Al-Tatayyur) 31 The Book of Manumission of Slaves 32 Dialects and Readings of the Qur'an (Kitab Al-Huruf Wa Al-Qira'at) 33 Hot Baths (Kitab Al-Hammam) 34 Clothing (Kitab Al-Libas) 35 Combing the Hair (Kitab Al-Tarajjul) 36 Signet-Rings (Kitab Al-Khatam) 37 Trials and Fierce Battles (Kitab Al-Fitan Wa Al-Malahim) 38 The Promised Deliverer (Kitab Al-Mahdi) 39 Battles (Kitab Al-Malahim) 40 Prescribed Punishments (Kitab Al-Hudud) 41 Types of Blood-Wit (Kitab Al-Diyat) 42 Model Behavior of the Prophet (Kitab Al-Sunnah) 43 General Behavior (Kitab Al-Adab)