You divorced (in a manner that is) against the Sunnah and took back (in a manner that is) against the Sunnah. Call someone to bear witness to her divorce and to her reunion, and do not do that again
Authentic hadithعن عمران بن حصين رضي الله عنهما أنه سُئِلَ عن الرجل يُطلِّق امرأته، ثم يَقَعُ بها، ولم يُشْهِدْ على طلاقها، ولا على رَجْعَتِهَا، فقال: "طَلَّقْتَ لِغَيْرِ سُنَّةٍ، وَرَاجَعْتَ لِغَيْرِ سُنَّةٍ، أَشْهِدْ على طلاقها، وعلى رَجْعَتِهَا، ولا تَعُدْ".
‘Imrān ibn Husayn (may Allah be pleased with him) reported that he was asked about a person who divorced his wife and then had intercourse with her without calling any witnesses to the divorce or the reunion. He said: "You divorced (in a manner that is) against the Sunnah and took back (in a manner that is) against the Sunnah. Call someone to bear witness to her divorce and to her reunion, and do not do that again."
Explanation
In this Hadīth, ‘Imrān ibn Husayn (may Allah be pleased with him) was asked about a man who divorced his wife and then had intercourse with her after he took her back in marriage, but without calling anybody to witness either divorce or reunion. ‘Imrān (may Allah be pleased with him) answered that such a husband has violated the Sunnah in both cases as he has not brought witnesses to the divorce or to the reunion. He ordered him to bring witnesses for the divorce and the reunion and to avoid doing so again.
Source · Narrated by Ibn Majah - Narrated by Abu Daoud
From the Hadith Encyclopedia (hadeethenc.com).