One-fifth of Booty to the Cause of Allah (Khumus)
Bukhari :: Book 4 :: Volume 53 :: Hadith 387
Narrated Abu Humaid As-Saidi:
We accompanied the Prophet in the Ghazwa of Tabuk and the king of 'Aila presented a white mule and a cloak as a gift to the Prophet. And the Prophet wrote to him a peace treaty allowing him to keep authority over his country.
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