Muqatil bin Hayyan (may Allah have mercy on him) said: In the Jahiliyyah, when a man died, the heirs of the deceased would go to his wife, throw a garment over her and thereby inherit her marriage, and that person would have the greater right to her. When Abu Qays bin al-Aslat (may Allah be pleased with him) died, his son approached his father's wife and married her, but had not yet had intercourse with her. That lady mentioned it to the Prophet (peace and blessings be upon him), and Allah Almighty revealed this verse concerning Qays: "And do not marry those women whom your fathers have married, except what has already passed." [Surah al-Nisa: 22] before the prohibition. Then the prohibition of mothers and daughters was mentioned: "And that you combine two sisters, except what has already passed" [Surah al-Nisa: 23] before the prohibition. "Indeed, Allah is ever Forgiving, Most Merciful." [Surah al-Nisa: 23] regarding what happened before the prohibition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13927