From Sahih Muslim (Hadith 319):
It is narrated on the authority of Abdullah b. Mas'ud:We once said: Messenger of Allah, would we be held responsible for our deeds committed in the state of ignorance? He (the Holy Prophet) observed: He who did good deeds in Islam would not be held responsible for what he did in the state of ignorance, but he who committed evil (after having come within the fold of Islam) would be held responsible for his previous and later deeds
Reference: Muslim 319