سورة Al-Baqarah ( The Cow ) - الآية 230

And if he has divorced her (the third time), then she is not lawful unto him thereafter until she has married another husband. Then, if the other husband divorces her, it is no sin on both of them that they reunite, provided they feel that they can keep the limits ordained by Allah. These are the limits of Allah, which He makes plain for the people who have knowledge.