If a woman apostatizes and her husband has intercourse with her on the same day before her repentance, then she repents the following day, is their intercourse considered unlawful sexual intercourse (zina)? Is the marriage contract annulled? Is she obligated to inform her husband of this? And is it permissible for her to remain with him?

The questioner did not specify what the wife did, nor whether it constituted apostasy. If it was apostasy, the husband bears no sin for having intercourse with his wife while unaware of what transpired from her, nor is it considered zina (fornication), nor does it invalidate the marriage contract. Since the wife repented on the second day, the marital bond remains intact, based on the preponderant opinion of the scholars regarding the cessation of separation between a husband and his apostate wife, which depends on the completion of the waiting period (iddah (definition)) before her repentance. If the wife repents before the iddah expires, the marriage remains valid. Al-Nawawi said: "If either of them apostatizes after consummation, the marriage is suspended until the wife's iddah expires. If the apostate among them returns to Islam before her iddah expires, they remain married."