A man asked Sayyiduna Imran bin Husayn (may Allah be pleased with him) about a man who 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 back against the Sunnah. Now let him take witnesses over his divorce and over his taking back, and let him seek forgiveness from Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15511