It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that he was asked about a man who divorced his wife without taking witnesses and took her back without taking witnesses. He said: If he divorced at an improper time and took her back contrary to the Sunnah, then let him take witnesses now.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15391