One-fifth of Booty to the Cause of Allah (Khumus)
Bukhari :: Book 4 :: Volume 53 :: Hadith 355
Narrated Abu Musa Al-Ashari:
A bedouin asked the Prophet, "A man may fight for the sake of booty, and another may fight so that he may be mentioned by the people, and a third may fight to show his position (i.e. bravery); which of these regarded as fighting in Allah's Cause?" The Prophet said, "He who fights so that Allah's Word (i.e. Islam) should be superior, fights for Allah's Cause."
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