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 f

'A'Asha (Allah be pleased with her) reported:

A person divorced his wife by three pronouncements; then another person married her and he also divorced her without having sexual intercourse with her. Then the first husband of her intended to remarry her. It was about such a case that Allah's Messenger (ﷺ) was asked, whereupon he said: No, until the second one has tasted her sweetness as the first one had tasted.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ، عَنْ عَائِشَةَ، قَالَتْ طَلَّقَ رَجُلٌ امْرَأَتَهُ ثَلاَثًا فَتَزَوَّجَهَا رَجُلٌ ثُمَّ طَلَّقَهَا قَبْلَ أَنْ يَدْخُلَ بِهَا فَأَرَادَ زَوْجُهَا الأَوَّلُ أَنْ يَتَزَوَّجَهَا فَسُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ ‏ "‏ لاَ حَتَّى يَذُوقَ الآخِرُ مِنْ عُسَيْلَتِهَا مَا ذَاقَ الأَوَّلُ ‏"‏ ‏.‏
Reference : Sahih Muslim 1433f
In-book reference : Book 16, Hadith 134
USC-MSA web (English) reference : Book 8, Hadith 3359
  (deprecated numbering scheme)