One-fifth of Booty to the Cause of Allah (Khumus)

Bukhari :: Book 4 :: Volume 53 :: Hadith 349

Narrated Abu Huraira:

Allah's Apostle said, "When Khosrau is ruined, there will be no Khosrau after him; and when Caesar is ruined, there will be no Caesar after him. By Him in Whose Hands my life is, you will spend their treasures in Allah's Cause."