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

وَحَدَّثَنَاهُ مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي ح، وَحَدَّثَنَاهُ مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى، - يَعْنِي ابْنَ سَعِيدٍ - جَمِيعًا عَنْ عُبَيْدِ اللَّهِ، بِهَذَا الإِسْنَادِ ‏.‏ مِثْلَهُ ‏.‏ وَفِي حَدِيثِ يَحْيَى عَنْ عُبَيْدِ اللَّهِ حَدَّثَنَا الْقَاسِمُ عَنْ عَائِشَةَ ‏.‏