Virtues and Merits of the Prophet (pbuh) and his Companions

Bukhari :: Book 4 :: Volume 56 :: Hadith 711

Narrated Abu Dhar:

The Prophet said, "If somebody claims to be the son of any other than his real father knowingly, he but disbelieves in Allah, and if somebody claims to belong to some folk to whom he does not belong, let such a person take his place in the (Hell) Fire."