Sayyiduna Imran bin Husayn (may Allah be pleased with him) was asked about a man who divorced his wife without calling witnesses, then took her back without calling witnesses (what is the ruling)? He said: "He divorced contrary to the 'iddah, and took her back contrary to the Sunnah; let him call witnesses from now on."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15420