Abu Dawud Book 13, Hadith Number 3010.

Narated By Abdullah ibn AbuBakr : Abdullah ibn AbuBakr and some children of Muhammad ibn Maslamah said: There remained some people of Khaybar and they confined themselves to the fortresses. They asked the Apostle of Allah (PBUH) to protect their lives and let them go. He did so. The people of Fadak heard this; they also adopted a similar way. (Fadak) was, therefore, exclusively reserved for the Apostle of Allah (PBUH), for it was not captured by the expedition of cavalry and camelry.