Bukhari

Hadith Commentary:
In the era of ignorance (Jahiliyyah), when people would adopt someone as a son, that son would attribute himself to the adoptive father instead of his biological father. Allah, the Exalted, has taken strict notice of this matter in Surah Al-Ahzab (: Al-Ahzab 33:5). Despite the prohibitive command, even today most people attribute an adopted child to themselves. However, this is absolutely not permitted in the Shari‘ah. Ibn Battal has written that if such fame (attribution) occurs unintentionally, it does not fall under the aforementioned warning. (: Fath al-Bari 12/67) In any case, the disbelief (kufr) mentioned here does not refer to real disbelief that expels a person from the fold of Islam; rather, what is meant is ingratitude for blessings. And Allah knows best.