Sayyiduna Imran bin Husayn (may Allah be pleased with him) was asked about a man who divorced and did not take witnesses, and took back (his wife) and did not take witnesses. He said: "He divorced against the Sunnah and took back contrary to the waiting period ('iddah). Now let him take witnesses."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15514