Obligatory Charity Tax (Zakat)

Bukhari :: Book 2 :: Volume 24 :: Hadith 490

Narrated Ibn Masud:

I heard the Prophet saying, "There is no envy except in two: a person whom Allah has given wealth and he spends it in the right way, and a person whom Allah has given wisdom (i.e. religious knowledge) and he gives his decisions accordingly and teaches it to the others."