Expiation for committing adultery with a married woman

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.