2:230 · Al-Baqara
فَإِنْ طَلَّقَهَا فَلَا تَحِلُّ لَهُ مِنْ بَعْدُ حَتَّىٰ تَنْكِحَ زَوْجًا غَيْرَهُ ۗ فَإِنْ طَلَّقَهَا فَلَا جُنَاحَ عَلَيْهِمَا أَنْ يَتَرَاجَعَا إِنْ ظَنَّا أَنْ يُقِيمَا حُدُودَ اللَّهِ ۗ وَتِلْكَ حُدُودُ اللَّهِ يُبَيِّنُهَا لِقَوْمٍ يَعْلَمُونَ
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.