Allah has not made for any man two hearts in his (one) body: nor has He made your wives whom ye divorce by Zihar your mothers: nor has He made your adopted sons your sons. Such is (only) your (manner of) speech by your mouths. But Allah tells (you) the Truth, and He shows the (right) Way.
This refers to a pre-Islamic custom among the pagan Arabs. A husband would say to
his wife, “You are henceforth as my mother’s back to me,” and thus removed himself from
conjugal relations with his wife. This was the equivalent of an irrevocable divorce, but a
woman thus divorced could not marry again. The Qur’ān’s forceful disapproval here was
the first step to abolishing this custom, soon followed by its explicit abolition in Sūrat al Mujādilah
In the pre-Islamic period, adopted and foster children were regarded as enjoying the
same legal status as biological children. This custom had other implications, notably that
the adopted child held the same status within the forbidden degrees of marriage as a biological child. Islam abolished this practice and declared that adoption has no legal effect.
(A father may not marry his natural son’s former wife, nor a son his natural father’s former
wife. The extension of that prohibition to adopted children is the specific occasion of the
abolition of the custom.)
Unal, Ali (2006). The Quran with Annotated Interpretation in Modern English. (surah 8 verse 41) http://www.islamicweblibrary.com/wp-content/uploads/2020/07/The-quran-with-annotated-interpretation-.pdf.