Abdullah bin Mubarak عبد الله بن المبارك · The Chapters on Divorce

was asked about a man who divorced his wife then had intercourse with her, and there were no witnesses to his divorcing her or his taking her back. 'Imran said: "You have divorced (her) in a manner that is not according to the Sunnah, and you have taken her back in a manner that is not according to the Sunnah. Bring people to witness your divorcing her and taking her back."

he was married to Umm Kulthum bint 'Uqbah, and she said to him when she was pregnant. "I will accept one divorce." So he divorced her once. Then he went out for prayer, and when he came back she had given birth. He said: "What is wrong with her? She misled me may Allah mislead her!" Then he came to the Prophet (ﷺ), who said: "Her waiting period is over (and she is divorced); propose marriage a new to her."

"Do not corrupt the Sunnah of our Prophet Muhammad (ﷺ). The waiting period of an Umm Walad is four months and ten (days)."