Imam al-Shafi'i (may Allah have mercy on him) said: "Because Allah, the Mighty and Majestic, made it unlawful only on account of the sanctity of the lawful (relationship), and the unlawful (relationship) is the opposite of the lawful."
He said: "And our view has been narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q13960
Yahya bin Ya'mar narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning a man who committed zina with his wife's mother or her daughter: he has transgressed two sanctities, but his wife does not become unlawful to him. Yahya bin Ya'mar said: The unlawful never makes the lawful unlawful. This reached al-Sha'bi (may Allah have mercy on him), and he said: Rather, if I took a jug of wine and poured it into a cistern of water, that water would become unlawful. Al-Sha'bi's view was that she does become unlawful to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13960
Ikrimah (may Allah have mercy on him) narrates from Abdullah bin Abbas (may Allah be pleased with them both) concerning a man who committed zina with his wife's mother; he said: He has transgressed two sanctities, but his wife does not become unlawful to him.
Al-Zuhri narrated from Ali (may Allah be pleased with him) the like of our view, and it is mursal (its chain is broken); it is also the view of Ibn al-Musayyib, Urwah and al-Zuhri.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13962
Uqayl narrates from Ibn Shihab that he was asked about a man who had committed zina with his wife's mother, and he said that Sayyiduna Ali (may Allah be pleased with him) said: "The unlawful does not make anything of the lawful unlawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13963
Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The unlawful does not make anything lawful unlawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13964
Sayyidah Aishah (may Allah be pleased with her) narrates from the Messenger of Allah (peace and blessings be upon him) that he said: "The unlawful does not make a lawful thing unlawful."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13965
Sayyidah Aishah (may Allah be pleased with her) narrates from the Messenger of Allah (peace and blessings be upon him) that the Messenger of Allah (peace and blessings be upon him) was asked about a man who commits zina with a woman: may he marry her daughter? Or who commits zina with her daughter: may he then marry her mother? The Messenger of Allah (peace and blessings be upon him) said: "The unlawful does not make the lawful unlawful; only what was through a lawful marriage makes (things) unlawful." Ishaq said: Abdullah bin Nafi' said: And this is what we adopt.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13966
Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "The lawful is not corrupted by the unlawful. Whoever commits zina with a woman, there is no harm on him if he marries her mother or her daughter; but as for (a relationship through) marriage, then no."
This Uthman bin Abd al-Rahman al-Waqqasi is alone in narrating it, and he is da'if (weak), as Yahya bin Ma'in and other hadith imams said. What is correct is from Ibn Shihab al-Zuhri from Ali (may Allah be pleased with him), mursal and mawquf (stopping at him). According to some scholars, the hadith of Abdullah al-Umari is better. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13967
Yunus bin Yazid (may Allah have mercy on him) said that Ibn Shihab (may Allah have mercy on him) was asked: If a man commits zina with a woman, may he marry her daughter? He said: Some scholars have said that Allah does not corrupt the lawful through the unlawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13968
As for what is narrated from Ibn Mas'ud (may Allah be pleased with him), that he said: "The unlawful and the lawful never come together without the unlawful prevailing over the lawful," only Jabir al-Ju'fi narrated it, from al-Sha'bi from Ibn Mas'ud. Jabir al-Ju'fi is da'if (weak), and al-Sha'bi from Ibn Mas'ud is munqati' (broken). Others narrated it, with its meaning, from al-Sha'bi as his own statement, not traced back to Abdullah bin Mas'ud.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13969