Ibn Sirin (may Allah have mercy on him) says that Imran bin Husayn (may Allah be pleased with him) was asked about a man who divorced his wife without making witnesses and took her back, also without making witnesses. Imran bin Husayn (may Allah be pleased with him) said: He divorced outside the (proper) waiting period and took her back in a way other than the Sunnah; let him make witnesses now.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15189