If a husband gets to know about his wife's past relations with other man. Is it permissible to divorce her then?
Though she is sincere and has changed her self and is faithful to her husband.
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And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and do not accept from them testimony ever after. And those are the defiantly disobedient. (4)
Except for those who repent thereafter and reform, for indeed, Allah is Forgiving and Merciful. (5)
And for those who accuse their wives, but have no witnesses except themselves, let the testimony of one of them be four testimonies (i.e. testifies four times) by Allah that he is one of those who speak the truth. (6)
And the fifth [oath will be] that the curse of Allah be upon him if he should be among the liars. (7)
But it will prevent punishment from her if she gives four testimonies [swearing] by Allah that indeed, he is of the liars. (8)
And the fifth [oath will be] that the wrath of Allah be upon her if he was of the truthful. (9)
And had it not been for the Grace of Allah and His Mercy on you. And that Allah is the One Who accepts repentance, the All-Wise. (10)
Qur'an Surah Noor (24).
Your question should have been is it must to have divorce?
But, if your question really is
is it permissable to divorce than ?
Yes, if both of the couple get into disagreement, disputes which they themselves couldn't solve, and if they're perfectly willing to break the marriage then, Yes.
But, if everything is going fine, no one is bothered much, and the wife maintains fidelity, then there is no need to go for divorce, and let the wife ask forgiveness from Allah.
Allah accepts only the repentance of those who do evil in ignorance and foolishness and repent soon afterwards; it is they to whom Allah will forgive and Allah is Ever All-Knower, All-Wise. (Qur'an 4:17)
May the creator guide us all.