Mutarrif (may Allah have mercy on him) said: Sayyiduna Imran bin Husayn (may Allah be pleased with him) was asked about a man who divorced his wife with one divorce and did not take witnesses, then took her back and did not take witnesses. Imran (may Allah be pleased with him) said: You divorced contrary to the Sunnah and you took her back contrary to the Sunnah; now take witnesses to her divorce and to her taking back.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15362