Call Them After Their Fathers

S 33:5 Call them by their fathers; it is more equitable with Allah. But if you have not known their fathers--then your brothers in the religion and your mawali. But there is no transgression in what you have sinned. But what your hearts intended. And Allah was a forgiver, merciful.

Adoption was abolished and children could no longer be named after the men taking them into their homes. So the man formerly known as Zayd bin Muhammad was then known as Zayd bin Harithah. In an Answering Islam article we read:

God has not assigned to any man two hearts within his breast; nor has He made your wives, when you divorce, saying, 'Be as my mother's back,' truly your mothers, neither has He made your adopted sons your sons in fact. That is your own saying, the words of your mouths; but God speaks the truth, and guides on the way. Call them after their true fathers; that is more equitable in the sight of God. If you know not who their fathers were, then they are your brothers in religion, and your clients. There is no fault in you if you make mistakes, but only in what your hearts premeditate. God is All-forgiving, All-compassionate. S. 33:4-5

Regarding this passage, Ibn Kathir says:

<nor has He made your adopted sons your real sons.> This was revealed concerning Zayd bin Harithah, may Allah be pleased with him, the freed servant of the Prophet. The Prophet had adopted him before prophethood, and he was known as Zayd bin Muhammad. Allah wanted to put an end to this naming and attribution ...

This is a command which abrogates the state of affairs that existed at the beginning of Islam, when it was permitted to call adopted sons after the man who adopted them. Then Allah commanded that they should be given back the names of their real fathers, and states that this was more fair and just. Al-Bukhari (may Allah have mercy on him) narrated that ‘Abdullah bin ‘Umar said: "Zayd bin Muhammad, may Allah be pleased with him, the freed servant of the Messenger of Allah was ALWAYS CALLED Zayd bin Muhammad, UNTIL (the words of the) QUR'AN WERE REVEALED ... <Call them (adopted sons) by (the names of) their fathers, that is more just with Allah.>"

This was also narrated by Muslim, At-Tirmidhi and An-Nasa’i. They used to deal with them as sons in every respect, including being alone with them as Mahrams and so on. Hence, Sahlah bint Suhaly, the wife of Abu Hudhayfah, may Allah be pleased with them both, said: "O Messenger of Allah! We used to call Salim our son, but Allah has revealed what He has revealed. He used to enter upon me, but I feel that Abu Hudhayfah does not like that." The Prophet said ...

((Breastfeed him and he will become your Mahram.)) Hence when this ruling was abrogated, Allah made it permissible for a man to marry the ex-wife of his adopted son, and the Messenger of Allah married Zaynab bint Jash, the divorced wife of Zayd bin Harithah, may Allah be pleased with him, Allah said ...

<So that (in the future) there may be no difficulty to the believers in respect of the wives of their adopted sons when the latter have no desire to keep them> (33:37) And Allah says in Ayat At-Tahrim ...

<The wives of your sons from your own loins> (4:23). The wife of an adopted son is not included because he was not born from the man's loins. A "foster" son through breastfeeding is the same as a son born from one's own loins, from the point of view of the Shari‘ah, because the Prophet said in the Two Sahihs ...

((Suckling makes unlawful as lineage does.)) ...

<Call them by their fathers.> This is concerning Zayd bin Harithah, may Allah be pleased with him. He was killed in 8 AH at the battle of Mu’tah. In Sahih Muslim it is reported that Anas bin Malik, may Allah be pleased with him, said: "The Messenger of Allah said ...

((O my son.))" It was also reported by Abu Dawud and At-Tirmidhi ...

<But if you know not their father's (sic) then they are your brothers in the religion and Mawalikum (your freed servants).>

Here Allah commands that adopted sons should be given back their fathers' names, if they are known; if they are not known, then they should be called brothers in faith or freed servants, to compensate for not knowing what their real lineage is. (Tafsir Ibn Kathir (Abridged) Volume 7 (Surat An-Nur to Surat Al-Ahzab, Verse 50), abridged by a group of scholars under the supervision of Shaykh Safiur-Rahman Al-Mubarakpuri [Darussalam Publishers & Distributors, Riyadh, Houston, New York, London, Lahore; First Edition: August 2000], pp. 634-637; bold and capital emphasis ours)

And, in reference to surah 33:40, Ibn Kathir writes:

<Muhammad is not the father of any of your men,>

After this it was not permitted to say Zayd bin Muhammad, i.e., he was not his father even though he had adopted him ... (Ibid., p. 701; bold emphasis ours)

Muslim jurist, Imam Malik, recorded:

Yahya related to me from Malik from Ibn Shihab that he was asked about the suckling of an older person. He said, ''Urwa ibn az-Zubayr informed me that Abu Hudhayfa ibn Utba ibn Rabia, one of the companions of the Messenger of Allah, may Allah bless him and grant him peace, who was present at Badr, adopted Salim (who is called Salim, the mawla of Abu Hudhayfa) as the Messenger of Allah, may Allah bless him and grant him peace, adopted Zayd ibn Haritha. He thought of him as his son, and Abu Hudhayfa married him to his brother's sister, Fatima bint al-Walid ibn Utba ibn Rabia, who was at that time among the first emigrants. She was one of the best unmarried women of the Quraysh. When Allah the Exalted sent down in His Book what He sent down about Zayd ibn Haritha, 'Call them after their true fathers. That is more equitable in the sight of Allah. If you do not know who their fathers were then they are your brothers in the deen and your mawali,' (Sura 33 ayat 5) people in this position were traced back to their fathers. When the father was not known, they were traced to their mawla... (Malik's Muwatta, Book 30, Number 30.2.12) Revisiting Muhammad's Marriage to His Adopted Son's Divorcee, Zaynab Bint Jash

If a child is to be named after his father, does it apply in the case of fornication?

Narrated `Aisha:

Sa`d bin Abi Waqqas and 'Abu bin Zam`a quarreled over a boy. Sa`d said, "O Allah's Messenger (ﷺ)! This boy is the son of my brother (`Utba bin Abi Waqqas) who took a promise from me that I would take him as he was his (illegal) son. Look at him and see whom he resembles." 'Abu bin Zam`a said, "O Allah's Messenger (ﷺ)! This is my brother and was born on my father's bed from his slave-girl." Allah's Apostle cast a look at the boy and found definite resemblance to `Utba and then said, "The boy is for you, O 'Abu bin Zam`a. The child goes to the owner of the bed and the adulterer gets nothing but the stones (despair, i.e. to be stoned to death). Then the Prophet (ﷺ) said, "O Sauda bint Zama! Screen yourself from this boy." So, Sauda never saw him again.

https://sunnah.com/bukhari:2218