A person who attributes his fatherhood to anyone other than his real father, knowing that he is not his father, commits an act of disbelief.

And he who makes a claim of anything which in fact does not belong to him, is none of us. He should make his abode in Hell,

and he who labels anyone as disbeliever or calls him the enemy of Allah and he is in fact not so, his charge will revert to him.

Bukhari and Muslim. Riyad-us-Saliheen (Hadith # 1805)