Surah Al-Baqarah (The Cow), Verse 230

فَاِنْ طَلَّقَهَا فَلَا تَحِلُّ لَهٗ مِنْ بَعْدُ حَتّٰی تَنْكِحَ زَوْجًا غَیْرَهٗ فَاِنْ طَلَّقَهَا فَلَا جُنَاحَ عَلَیْهِمَاۤ اَنْ یَّتَرَاجَعَاۤ اِنْ ظَنَّاۤ اَنْ یُّقِیْمَا حُدُوْدَ اللّٰهِ وَتِلْكَ حُدُوْدُ اللّٰهِ یُبَیِّنُهَا لِقَوْمٍ یَّعْلَمُوْنَ ۟

Fa in tallaqahaa falaa tahillu lahoo mim ba'du hattaa tankiha zawjan ghairah; fa in tallaqahaa falaa junaaha 'alaihimaaa ai yataraaja'aaa in zannaaa ai yuqeemaa hudoodal laa; wa tilka hudoodul laahi yubaiyinuhaa liqawminy ya'lamoon

“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.”

Quran 2:230 (Yusuf Ali Translation)