One of the most tragic and consequential sins committed is zina (adultery or fornication). This heinous crime destroys families and lives and destroys in some people the hope of attaining Allah’s Pleasure. But in Islam, the door of repentance is always open, only that the repentance itself demands a resolve to cease from a sinful activity. A person who has engaged in an adulterous relationship must resolve to cease from the sinful sexual activity and return to Allah, seeking His Forgiveness. Allah will surely turn to him or her and shower him/her with His Mercy.
This indicates that the one who commits zina should never lose hope of gaining Allah’s Mercy. He or she should repent very seriously and immediately, rushing to Allah to seek His Forgiveness.
Allah says in the Qur’an: “Say: ‘O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.” (Az-Zumar 53)
Moreover, the Glorious Qur’an gives glad tidings to those penitent people who have committed sins such as zina, regretted their sins, determined not to repeat them, and performed righteous deeds that Allah will replace their sins with hasanat or rewards. Allah says, “And true servants of Allah are those who cry not unto any other god along with Allah, nor take the life which Allah hath forbidden save in (course of) justice, nor commit adultery and whoso doeth this shall pay the penalty; the doom will be doubled for him on the Day of Resurrection, and he will abide therein disdained for ever; save him who repenteth and believeth and doth righteous work; as for such, Allah will change their evil deeds to good deeds. Allah is ever Forgiving, Merciful. And, whosoever repenteth and doeth good, he verily repenteth toward Allah with true repentance.” (Al-Furqan 68-71)

SheikhHussainHalawa, the secretary general of the European Council for Fatwa and Research (ECFR) and imam of the Islamic Cultural Center of Ireland (ICCI) states the following: “As for his marriage with her daughter, if it meets the conditions of Islamic marriage, it is held as valid marriage. As for the illegal relations, they both should repent to Allah. I pray to Allah to forgive us all.”