Imran bin Husain:was asked about a man who divorced his wife then had intercourse with her, and there were no witnesses to his divorcing her or his taking her back. 'Imran said: "You have divorced (her) in a manner that is not according to the Sunnah, and you have taken her back in a manner that is not according to the Sunnah. Bring people to witness your divorcing her and taking her back

English translation · Sunan Ibn Majah