Nor has He made your adopted sons your (real) sons; that is simply a saying of your mouths. But Allah speaks the truth and guides you to the (right way). Call them by their fathers names, that is more just in the sight of Allah. But if you do not know their fathers, they are your brothers in faith and your ward...(33:4-5)

check this also
http://www.islamonline.net/servlet/Satellite?pagename=IslamOnline-Englis...