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

Bukhari :: Book 4 :: Volume 53 :: Hadith 329

Narrated Aisha:

Allah's Apostle died, and there was nothing in my house that a living being could eat, except some barley Lying on a shelf. So, I ate of it for a long period and measured it, and (after a short period) it was consumed.