Narrated 'Imran bin Husain (RA):

He was asked about a man who divorces his wife, then takes her back without taking witnesses (on either). So he replied, "Get witnesses on her divorce, and on her return." [Abu Dawud reported it Mawquf (saying of a Companion), and its chain of narrators is Sahih (authentic)].

عَنْ عِمْرَانَ بْنِ حُصَيْنٍ رَضِيَ اَللَّهُ عَنْهُمَا; { أَنَّهُ سُئِلَ عَنْ اَلرَّجُلِ يُطَلِّقُ, ثُمَّ يُرَاجِعُ, وَلَا يُشْهِدُ? فَقَالَ: أَشْهِدْ عَلَى طَلَاقِهَا, وَعَلَى رَجْعَتِهَا } .‏ رَوَاهُ أَبُو دَاوُدَ هَكَذَا مَوْقُوفًا, وَسَنَدُهُ صَحِيحٌ .‏ [1]‏ .‏