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

Bukhari :: Book 4 :: Volume 53 :: Hadith 366

Narrated Jabir bin Abdullah:

While Allah's Apostle was distributing the booty at Al-Ja'rana, somebody said to him "Be just (in your distribution)." The Prophet replied, "Verily I would be miserable if I did not act justly."