A man asked Sayyiduna Imran bin Husayn (may Allah be pleased with him): A man divorced and did not take witnesses, and took back (his wife) and did not take witnesses (so what is the ruling)? He said: "He did an evil thing. He divorced according to innovation (bid'ah) and took back against the Sunnah. Now let him take witnesses over what he has done."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15510