Musnad Ahmad 27:497

Hammad ibn Usamah narrated to us, he said: Ubaydullah [narrated to me]: Nafi’ informed me, from Ibn Umar, "That the Messenger of Allah (ﷺ) made a contract with the people of Khaybar for half of what came forth from crops or dates. He used to give his wives every year one hundred wasqs: eighty wasqs of dates, and twenty wasqs of barley."