Hadith 13:12
Narrated Mutarrif ibn Abdullah:
Imran ibn Husayn was asked about a person who divorces his wife, and then has intercourse with her, but he does not call any witness to her divorce nor to her restoration. He said: You divorced against the sunnah and took her back against the sunnah. Call someone to bear witness to her divorce, and to her return in marriage, and do not repeat it.
حَدَّثَنَا بِشْرُ بْنُ هِلاَلٍ، أَنَّ جَعْفَرَ بْنَ سُلَيْمَانَ، حَدَّثَهُمْ عَنْ يَزِيدَ الرِّشْكِ، عَنْ مُطَرِّفِ بْنِ عَبْدِ اللَّهِ، أَنَّ عِمْرَانَ بْنَ حُصَيْنٍ، سُئِلَ عَنِ الرَّجُلِ، يُطَلِّقُ امْرَأَتَهُ ثُمَّ يَقَعُ بِهَا وَلَمْ يُشْهِدْ عَلَى طَلاَقِهَا وَلاَ عَلَى رَجْعَتِهَا فَقَالَ طَلَّقْتَ لِغَيْرِ سُنَّةٍ . وَرَاجَعْتَ لِغَيْرِ سُنَّةٍ أَشْهِدْ عَلَى طَلاَقِهَا وَعَلَى رَجْعَتِهَا وَلاَ تَعُدْ
Original Reference: Sunan Abi Dawud 2186