كتاب النكاح
16  
The Book of Marriage
(17)
Chapter: 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
(17)
باب لاَ تَحِلُّ الْمُطَلَّقَةُ ثَلاَثًا لِمُطَلِّقِهَا حَتَّى تَنْكِحَ زَوْجًا غَيْرَهُ وَيَطَأَهَا ثُمَّ يُفَارِقَهَا وَتَنْقَضِي عِدَّتُهَا
Sahih Muslim 1433 d

'A'isha (Allah be pleased with her) reported that Allah's Messenger (ﷺ) was asked about a woman whom a man married and then divorced her, and then she married (another) person, and she was divorced before sexual intercourse with her, whether it was lawful for her first husband (to marry her in this state). He (the Holy Prophet) said:

No, until he has tasted her sweetness.
حَدَّثَنَا مُحَمَّدُ بْنُ الْعَلاَءِ الْهَمْدَانِيُّ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ عَنِ الْمَرْأَةِ يَتَزَوَّجُهَا الرَّجُلُ فَيُطَلِّقُهَا فَتَتَزَوَّجُ رَجُلاً فَيُطَلِّقُهَا قَبْلَ أَنْ يَدْخُلَ بِهَا أَتَحِلُّ لِزَوْجِهَا الأَوَّلِ قَالَ ‏ "‏ لاَ حَتَّى يَذُوقَ عُسَيْلَتَهَا ‏"‏‏.‏
Reference : Sahih Muslim 1433d
In-book reference : Book 16, Hadith 132
USC-MSA web (English) reference : Book 8, Hadith 3357
  (deprecated numbering scheme)