33: The Clans
What verse in the Quran made adoption haram (forbidden) in Islam?
Quran 33:5
Proclaim their real parentage. That will be more equitable in the sight of Allah. And if ye know not their fathers, then (they are) your brethren in the faith, and your clients. And there is no sin for you in the mistakes that ye make unintentionally, but what your hearts purpose (that will be a sin for you). Allah is ever Forgiving, Merciful. Quran 33:5
The verse was revealed to justify Muhammad's marriage to the wife of his adopted son.
See also Quran 33:37-40