Sahih Bukhari
One-fifth of Booty to the Cause of Allah (Khumus)
Volume: 004 / Book Number: 053 / Hadith Number: 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."