Musannaf Ibn Abi Shayba 13:1272

Abu Bakr narrated to us, he said: Ibn Abi Za'idah narrated to us, from Hajjaj, from Abu Ja'far, he said: The Messenger of Allah (may Allah bless him and grant him peace) made a contract with the people of Khaybar for half [of the produce], then Abu Bakr, and 'Uthman, and 'Ali [did the same]. Then their families, until today, give a third and a quarter.