Ruling on giving one's name to a foundling whom
    one adopts
Question

                A person was not blessed with
      children, so he took a foundling from the orphanage and brought him up, teaching him and
      treating him well. Out of compassion and an attempt to the do right thing, he gave him his
      name and registered him as his own son in official documents. What is the ruling on this?
Praise be to Allah.According to shareeah it is not
    permissible for a person who adopts a child to give him his name as if he is his own son
    and a member of his tribe, as described in the question, because this involves lying,
    falsehood, the mixing of lineages and a threat to peoples honour, as well as
    distorting the channels of inheritance so that someone who is not entitled may receive an
    unfair share whilst depriving the rightful heirs of their due, and causing chaos in the
    sphere of relationships between mahram and non-mahram which affect issues of khulwah
    (being alone with a member of the opposite sex) and marriage. All of this violates
    sanctities and transgresses the bounds of shareeah. Hence Allaah forbade saying that
    a child belongs to anyone except his father or a slave belongs to anyone except his
    master. As Allaah said (interpretation of the meaning):
nor has He made your adopted sons your real sons.
    That is but your saying with your mouths. But Allâh says the truth, and He guides to the
    (Right) Way. Call them (adopted sons) by (the names of) their fathers, that is more just
    with Allâh. But if you know not their fathers (names, call them) your brothers in
    Faith and Mawâlîkum (your freed slaves). And there is no sin on you concerning that in
    which you made a mistake, except in regard to what your hearts deliberately intend. And
    Allâh is Ever Oft-Forgiving, Most Merciful. [al-Ahzaab 33:4-5]
The Prophet 
 (peace and blessings of Allaah be upon him)
    said:
Whoever knowingly claims to belong to anyone other than
    his father, Paradise will be denied him. (Narrated by Ahmad, al-Bukhaari and
    Muslim). The Prophet 
 (peace and blessings of Allaah be upon him) also said: Whoever
    claims to be the son of anyone other than his father, or the slave of anyone other than
    his master, on him will be the curse of Allaah continually until the Day of
    Resurrection. 
The one who is asking this question made a mistake when he
    tried to do the right thing by giving his name to the child. He cannot leave matters as
    they are; he has to try to change it and put matters right, because of the texts that
    state this naming is haraam, as quoted above. As for his feeling compassion for the child,
    bringing him up and treating him kindly, this is a good action which is encouraged in
    Islam. May Allaah bless our prophet Muhammad.