Sahih Bukhari Volume 9, Book 84, Hadith Number 56.

Narrated By Ibn Mas’ud : A man said, “O Allah’s Apostle! Shall we be punished for what we did in the Prelslamic Period of ignorance?” The Prophet said, “Whoever does good in Islam will not be punished for what he did in the Pre-lslamic Period of ignorance and whoever does evil in Islam will be punished for his former and later (bad deeds).”