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."

commentary:
one of the customs in the pre-islamic era of ignorance was that
whenever a person passed away, his offsprings 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 (p.b.u.h.). when she
went to the prophet (p.b.u.h.) and told him the matter, this verse was
revealed and prohibited the marriage with step mother.

" 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."