Expiation for committing adultery with a married woman
Question

                What is the expiation for one who committed adultery with a married woman?.
Praise be to Allah.Zina (adultery) is 
      nothing but evil. But with a married woman it is a greater sin because 
      that is a transgression against the husband’s honour. Allaah says 
      concerning zina (interpretation of the meaning): 

    “And come not near to unlawful sex. Verily, it is a 
    Faahishah (i.e. anything that transgresses its limits: a great sin), and an 
    evil way (that leads one to hell unless Allaah forgives him)”

    [al-Isra’ 17:32]
The Prophet 
     (peace and blessings of Allaah be upon him) said: “If a man commits 
    zina, faith will come out of him until it becomes like a cloud over him, and 
    when he stops, his faith comes back to him.” Narrated by Abu Dawood, 4690; 
    al-Tirmidhi, 2625; classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

    Ibn Hajar al-Haythami said in al-Zawaajir ‘an Iqtiraaf 
    al-Kabaa’ir (2/138): Thus it is also known that there are different 
    categories of zina. If it is done with a non-mahram woman who has no husband 
    it is a serious matter. It is more serious if it is with a non-mahram woman 
    who has a husband; it is more serious still if it is with a mahram. The zina 
    of a previously-married person is worse than zina of a virgin, which is 
    indicated by the fact that the punishment is different. And the zina of an 
    old man who is of sound mind is worse than the zina of a young man, and the 
    zina of a free man or a knowledgeable man is worse than the zina of a slave 
    or one who is ignorant. End quote. 

    The expiation for committing zina with a married woman or any 
    other is sincere repentance that fulfils all the conditions of repentance. 
    That involves giving up the sin completely, regretting what one has done and 
    resolving never to go back to it. Whoever does that has repented to Allaah, 
    and whoever repents, Allaah will accept his repentance and turn his bad 
    deeds into good deeds, as Allaah says (interpretation of the meaning): 

    “Know they not that Allaah accepts repentance from His 
    slaves and takes the Sadaqaat (alms, charity), and that Allaah Alone is the 
    One Who forgives and accepts repentance, Most Merciful?”

    [al-Tawbah 9:104] 

    “And those who invoke not any other ilaah (god) along with 
    Allaah, nor kill such person as Allaah has forbidden, except for just cause, 
    nor commit illegal sexual intercourse  
    and whoever does this shall receive the punishment.

    69. The torment will be doubled to him on the Day of 
    Resurrection, and he will abide therein in disgrace;

    70. Except those who repent and believe (in Islamic 
    Monotheism), and do righteous deeds; for those, Allaah will change their 
    sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

    [al-Furqaan 25:68-70]

    Al-Bukhaari (4436) and Muslim (174) narrated from Ibn ‘Abbaas 
    (may Allaah be pleased with him) that some people from among the mushrikeen 
    who had killed and killed a great deal, and committed zina and done it a 
    great deal came to Muhammad  (peace and blessings of Allaah be upon him) 
    and said: “What you are saying and calling us to is good, if only you could 
    tell us that there is any expiation for what we have done.” Then the words
    “And those who invoke not any other ilaah (god) along with Allaah, nor 
    kill such person as Allaah has forbidden, except for just cause, nor commit 
    illegal sexual intercourse” [al-Furqaan 25:68] and “Say: O ‘Ibaadi 
    (My slaves) who have transgressed against themselves (by committing evil 
    deeds and sins)! Despair not of the Mercy of Allaah” [al-Zumar 39:52] 
    were revealed. 

    Whoever falls into zina must hasten to repent to Allaah and 
    conceal himself with His concealment, and not disclose what he has done, 
    because the Prophet  (peace and blessings of Allaah be upon him) said: 
    “Avoid this filthy thing that Allaah has forbidden, and whoever commits any 
    sin of that nature let him conceal himself with the concealment of Allaah.” 
    Narrated by al-Bayhaqi and classed as saheeh by al-Albaani in al-Silsilah 
    al-Saheehah, 663. 
Muslim (2590) narrated 
    from Abu Hurayrah (may Allaah be pleased with him) that the Prophet  
    (peace and blessings of Allaah be upon him) said: “Allaah does not cover any 
    slave in this world but Allaah will also cover him on the Day of 
    Resurrection.” 

    And Allaah knows best.