Sura al-Baqara 2:230

فَإِن طَلَّقَهَا فَلَا تَحِلُّ لَهُۥ مِنۢ بَعْدُ حَتَّىٰ تَنكِحَ زَوْجًا غَيْرَهُۥ فَإِن طَلَّقَهَا فَلَا جُنَاحَ عَلَيْهِمَآ أَن يَتَرَاجَعَآ إِن ظَنَّآ أَن يُقِيمَا حُدُودَ ٱللَّـهِ وَتِلْكَ حُدُودُ ٱللَّـهِ يُبَيِّنُهَا لِقَوْمٍ يَعْلَمُونَ

fa-in ṭallaqahā fa-lā taḥillu lahu min baʿdu ḥattā tankiḥa zawjan ghayrahu fa-in ṭallaqahā fa-lā junāḥa ʿalayhimā an yatarājaʿā in ẓannā an yuqīmā ḥudūda allāhi wa-tilka ḥudūdu allāhi yubayyinuhā li-qawmin yaʿlamūna

So if a husband divorces his wife (irrevocably), He cannot, after that, re-marry her until after she has married another husband and He has divorced her. In that case there is no blame on either of them if they re-unite, provided they feel that they can keep the limits ordained by Allah. Such are the limits ordained by Allah, which He makes plain to those who understand.