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.