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wa-lā tankiḥū mā nakaḥa ʾābāʾukum mina n-nisāʾi ʾillā mā qad salafa ʾinnahū kāna fāḥishatan wa-maqtan wa-sāʾa sabīlan
Do not marry any of the women whom your fathers had married, excluding what is already past. That is indeed an indecency, an outrage and an evil course.
— Ali Quli Qarai
Surah An-Nisa',Verse 22
22. "And do not marry women whom your father married, except what has already passed; verily it is indecent and hateful and it is an evil way."
One of the customs in the pre-Islamic era of Ignorance was that whenever a person passed away, his offspring would marry their step mother (their fathers' wife). It happened that one of the Helpers, called Abu-Qays, passed away and his son suggested to marry his step mother. The woman said that she should ask the matter from the Messenger of Allah (S). When she went to the Prophet (S) and told him the matter, this verse was revealed and prohibited the marriage with step mother.