Surah Al-Ahzaab (33) — Ayah 4

Allah has not placed two hearts [2] in any man's chest. Nor has He made your wives whom you declare to be like your mothers your actual mothers, nor has He made your adopted sons [3] your real sons. These are only words from your mouths, but Allah speaks the truth and He alone shows the right way.

[2]
The Meaning of Allah Not Making Two Hearts:

In this verse, Allah Almighty has stated three facts. The first is that Allah has made only one heart for all humans, not two. Therefore, one heart can accept only one reality at a time. It cannot accept two contradictory realities. It is impossible for a person to be a believer and a hypocrite at the same time, or to be a disbeliever and a Muslim simultaneously. Out of these, only one reality can be accepted at a time, because out of two contradictory things, only one can be true. And if a hypocrite hides hypocrisy in his heart while claiming faith, he is not stating the truth. Rather, he is lying and deceiving, and as a result of this marriage, Muslims will also come to know how many among them are sincere Muslims and who they are, and who are the hypocrites who, contrary to Allah’s command, speak out just like other enemies.

The Custom of Zihar is Invalid, as is the Custom of Adoption, Because There Can Only Be One Mother and One Father:

Among the Arabs, besides divorce, there was also a custom that if a man was angry with his wife or had a dispute with her, he would say, “You are to me like the back of my mother.” This act was called zihar. Upon saying this, the woman would become forbidden to her husband. This custom also needed reform. The method of its correction and its expiation has been explained in detail in Surah Mujadilah. Here, it is only being stated that by your saying so, your wife does not become your mother. A mother is only she who gave birth. So just as a person cannot have two hearts, similarly, no one can have two mothers. Now, if you call your wife your mother and actually consider her your mother, this is merely a statement from your mouth, which has nothing to do with reality.

Reform of the Custom of Adoption and Inheritance:

And the third fact is that an adopted son cannot take the place of a real son. It is also impossible for someone to have two fathers. The father is only he from whose seed he was born. No other person can be his real father, nor can he be his real son. An adopted son cannot inherit from his adoptive father, nor can the father inherit from the adopted son. Every person can marry the divorced or widowed wife of his adopted son. Similarly, the adopted son can marry the divorced or widowed wife of his adoptive father. And this was the wrong custom prevalent in society, which Allah Almighty wanted to reform through the marriage of the Prophet ﷺ with Lady Zainab.
[3] In this verse, some details of this reformative step are being stated. The first thing is that every person should be called by reference to his real father. Thus, Sayyiduna Abdullah bin Umar ؓ says that when the Messenger of Allah ﷺ made Sayyiduna Zaid his adopted son, we used to call him Zaid bin Muhammad. When this verse was revealed, we started calling him Zaid bin Harithah. [بخاري۔ كتاب التفسير]
And the Messenger of Allah ﷺ himself also began to call him Zaid bin Harithah. Incidentally, from this verse it is also understood that Sayyiduna Isa had no father. Otherwise, Allah Almighty would never have said Isa ibn Maryam. For the deniers of the miraculous birth of Sayyiduna Isa ibn Maryam, this is a point to ponder, because the Quran has not mentioned the name of any man or woman with filiation except him.