Sahih Bukhari Volume 004, Book 055, Hadith Number 572.
Narated By Abu Huraira : The people said, "O Allah's Apostle! Who is the most honorable amongst the people (in Allah's Sight)?" He said, "The most righteous amongst them." They said, "We do not ask you, about this. " He said, "Then Joseph, Allah's Prophet, the son of Allah's Prophet, The son of Allah's Prophet the son of Allah's Khalil (i.e. Abraham)." They said, "We do not want to ask about this," He said' "Then you want to ask about the descent of the Arabs. Those who were the best in the pre-lslamic period of ignorance will be the best in Islam provided they comprehend the religious knowledge."