It is not permissible for a woman who has been thrice-divorced to return to the one who divorced her until she marries another husband who has intercourse with her, then divorces her, and she completes the 'Iddah
حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ رِفَاعَةَ الْقُرَظِيَّ، طَلَّقَ امْرَأَتَهُ فَتَزَوَّجَهَا عَبْدُ الرَّحْمَنِ بْنُ الزَّبِيرِ فَجَاءَتِ النَّبِيَّ صلى الله عليه وسلم فَقَالَتْ يَا رَسُولَ اللَّهِ إِنَّ رِفَاعَةَ طَلَّقَهَا آخِرَ ثَلاَثِ تَطْلِيقَاتٍ . بِمِثْلِ حَدِيثِ يُونُسَ .
`A'isha (Allah be pleased with her) reported that Rifa`a al Qurazi divorced his wife and afterwards `Abd al-Rahman b. al-Zubair married her. She came to Allah's Messenger (ﷺ) and said: Messenger of Allah, Rifa`a has divorced me by three pronouncements. (The rest of the hadith is the same.)