Wala tankihoo ma nakaha abaokum mina annisa-i illa ma qad salafa innahu (surah Nisa: 22)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(4:22) Do not marry the women whom your fathers married, although what is past is past. *32 This indeed was a shameful deed, a hateful thing, and an evil way. *33

Wa laa tankihoo maa nakaha aabaaa`ukum minan nisaaa`i illaa maa qad salaf; inahoo kaana faahishatanw wa maqtanw wa saaa`a sabeelaa

*32). The Qur'an rounds off all statements prohibiting the objectionable features of the social life of the Jahiliyah period by condoning violations of those prohibitions prior to their revelation: 'What is past is past.' This has two meanings. First, that those concerned would not be punished for mistakes committed in their state of Ignorance, providing they rectified their conduct after the prohibitory injunction had been revealed. Second, that the prohibition of any ancient custom, usage and law did not mean that all acts which took place in the past would be nullified, and that all the consequences of those acts would be deemed void, and people absolved of all the obligations which ensued from them. If marriage with the step-mother, for instance, was prohibited it did not necessarily follow that the children of all such marriages which had been contracted in the past were to be reckoned illegitimate, and that the offspring from such marriages would be disinherited. Similarly, if a certain transaction was declared unlawful it did not mean that all such transactions which had taken place prior to the prohibition should be deemed void and that all the earnings of people accumulated through those transactions would be either seized or declared illegitimate property.
*33). In Islamic law marrying women who fall in the prohibited degrees of marriage is a recognized criminal offence. According to traditions in the Hadith collections of Abu Da'ud, Nasa'i and Ahmad b. Hanbal, people guilty of this offence were punished by the Prophet (peace be on him) with death and confiscation of property. It appears from the tradition related by Ibn 'Abbas (found in the collection of Ibn Majah), that the Prophet (peace be on him) had devised the following general rule: 'Kill whosoever commits sexual intercourse with a woman forbidden to him' (Ibn Majah; 'Hudud', 13, 35; also Ahmad b. Hanbal, Musnad, vol. 1, p. 300 - Ed.) There is some disagreement, however, among jurists on this question. Ahmad b. Hanbal is of the opinion that the convicted person should be put to death and his property confiscated. Abu Hanifah, Malik and Shafi'i are of the opinion that if a person commits sexual intercourse with a woman within the prohibited degrees he should be punished for adultery; and if he merely marries (but has not actually had sexual intercourse - Ed.) he should be subjected to severe punishment.