'Imran bin Husain: 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."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا بِشْرُ بْنُ هِلاَلٍ الصَّوَّافُ، حَدَّثَنَا جَعْفَرُ بْنُ سُلَيْمَانَ الضُّبَعِيُّ، عَنْ يَزِيدَ الرِّشْكِ، عَنْ مُطَرِّفِ بْنِ عَبْدِ اللَّهِ بْنِ الشِّخِّيرِ، أَنَّ عِمْرَانَ بْنَ الْحُصَيْنِ، سُئِلَ عَنْ رَجُلٍ، يُطَلِّقُ امْرَأَتَهُ ثُمَّ يَقَعُ بِهَا وَلَمْ يُشْهِدْ عَلَى طَلاَقِهَا وَلاَ عَلَى رَجْعَتِهَا . فَقَالَ عِمْرَانُ طَلَّقْتَ بِغَيْرِ سُنَّةٍ وَرَاجَعْتَ بِغَيْرِ سُنَّةٍ أَشْهِدْ عَلَى طَلاَقِهَا وَعَلَى رَجْعَتِهَا .
sunnah.com