Al-Baqarah 2:230

Fa-in tallaqaha falatahillu lahu min baAAdu hatta tankihazawjan ghayrahu fa-in tallaqaha fala junahaAAalayhima an yatarajaAAa in thannaan yuqeema hudooda Allahi watilka hudooduAllahi yubayyinuha liqawmin yaAAlamoon

And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him. And if he [i.e., the latter husband] divorces her [or dies], there is no blame upon them [i.e., the woman and her former husband] for returning to each other if they think that they can keep [within] the limits of Allāh. These are the limits of Allāh, which He makes clear to a people who know [i.e.,understand].

Thereafter, if he divorces her, she shall no longer remain lawful for him unless she marries a man other than him. Should he too divorce her, then there is no sin on them in their returning to each other, if they think they would maintain the limits set by Allah. These are the limits set by Allah that He makes clear to a people who know (that Allah is alone capable of setting these limits).