It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that a man divorced and did not take witnesses, and took back his wife and did not take witnesses. He said: "He divorced contrary to the waiting period (iddah), and took her back contrary to the Sunnah; now let him take witnesses to his divorce and to his taking her back."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15635