Musannaf Ibn Abi Shayba 28:756

Yahya ibn Adam narrated to us, from Zuhayr, from al-Hasan ibn al-Hurr, from al-Hakam, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) used to grant extra shares (nafal) before the obligation of the khums (one-fifth) in the spoils of war was revealed. When the verse, "And know that anything of booty that you take, then to Allah belongs its fifth," was revealed, he abandoned the extra shares that he used to grant. And it became, regarding that, the fifth of the fifth, which is the share of Allah and the share of the Prophet (may Allah bless him and grant him peace).