(12) باب عِتْقِ الْمُشْرِكِ
Narrated Hisham:
My father told me that Hakim bin Hizam manumitted one-hundred slaves in the Pre-Islamic period of ignorance and slaughtered one-hundred camels (and distributed them in charity). When he embraced Islam he again slaughtered one-hundred camels and manumitted one-hundred slaves. Hakim said, "I asked Allah's Messenger (ﷺ), 'O Allah's Messenger (ﷺ)! What do you think about some good deeds I used to practice in the Pre-Islamic period of ignorance regarding them as deeds of righteousness?' Allah's Apostle said, "You have embraced Islam along with all those good deeds you did."
Reference: Sahih al-Bukhari 2538
In-book reference: Book 49, Hadith 23
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 46, Hadith 715