Obligatory Charity Tax (Zakat)

Bukhari :: Book 2 :: Volume 24 :: Hadith 500

Narrated Abu Huraira:

A man came to the Prophet and asked, "O Allah's Apostle! Which charity is the most superior in reward?" He replied, "The charity which you practice while you are healthy, niggardly and afraid of poverty and wish to become wealthy. Do not delay it to the time of approaching death and then say, 'Give so much to such and such, and so much to such and such.' And it has already belonged to such and such (as it is too late)."