The Quran

ما جَعَلَ اللَّهُ لِرَجُلٍ مِن قَلبَينِ في جَوفِهِ ۚ وَما جَعَلَ أَزواجَكُمُ اللّائي تُظاهِرونَ مِنهُنَّ أُمَّهاتِكُم ۚ وَما جَعَلَ أَدعِياءَكُم أَبناءَكُم ۚ ذٰلِكُم قَولُكُم بِأَفواهِكُم ۖ وَاللَّهُ يَقولُ الحَقَّ وَهُوَ يَهدِي السَّبيلَ

Ali Unal

God has not made for any man two hearts within his body (one to be assigned for belief in and worship of Him, and the other to belief in and worship of others). Nor has He made your wives whom you may declare to be unlawful to you by using against them the expression, “Be as my mother’s back to me!” (to mean you divorce them¹), your mothers (in fact). Nor has He made your adopted sons your sons (in fact).² Those are only expressions you utter with your mouths, whereas God speaks the truth and He guides to 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.)