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