Musannaf Ibn Abi Shayba 28:560

'Adi ibn Yunus narrated to us, from Ibn 'Awn and Hisham, from Ibn Sirin, from Anas ibn Malik, who said: Ibn 'Awn said: Al-Bara' ibn Malik engaged in single combat with the Marzban of al-Za'rah on the day of al-Za'rah. Hisham said: Al-Bara' ibn Malik charged at the Marzban of al-Za'rah on the day of al-Za'rah and struck him with a spear thrust that broke the pommel of his saddle, so he killed him and took his two bracelets and his belt as spoils. When we arrived, 'Umar prayed the morning prayer, then he came to us and said: "Is Abu Talhah here?" He went out to him, and he ('Umar) said: "We used not to take a fifth of the spoils, but the spoils of al-Bara' are wealth, so if we take a fifth of it, it would reach six thousand, [meaning the total] reached thirty thousand." Muhammad said: Anas ibn Malik narrated to me that it was the first spoils [of war] to be subjected to the one-fifth tax (khums) in Islam.