Obligatory Charity Tax (Zakat)

Bukhari :: Book 2 :: Volume 24 :: Hadith 517

Narrated Hakim bin Hizam:

I said to Allah's Apostle, "Before embracing Islam I used to do good deeds like giving in charity, slave-manumitting, and the keeping of good relations with Kith and kin. Shall I be rewarded for those deeds?" The Prophet replied, "You became Muslim with all those good deeds (Without losing their reward)."