Musannaf Ibn Abi Shayba 28:561

'Abd al-Rahim ibn Sulayman narrated to us, from Hisham ibn Hassan, from Ibn Sirin, from Anas ibn Malik, who said: The spoils of the slain (al-salab) were not subject to the one-fifth tax (khums). The first spoils of the slain to be taxed in Islam were the spoils of al-Bara' ibn Malik. He had charged at the Marzban of al-Za'arah and pierced him with his spear until he broke the pommel of the saddle. Then he descended to him and cut off his belt and his two bracelets. He said: When we arrived in Medina, Umar ibn al-Khattab performed the morning prayer, then came to us and said, "Peace be upon you. Is Abu Talha there?" He said, "Yes." So he went out to him. Umar said, "We used to not tax the spoils of the slain, but the spoils of al-Bara' ibn Malik are wealth, and I am going to tax them." So he called the appraisers, and they appraised them at thirty thousand, and he took six thousand from them.