Musannaf Ibn Abi Shayba 28:568

Ad-Dahhak ibn Makhlad narrated to us, from al-Awza'i, from Ibn Shihab, from al-Qasim, who said: Ibn Abbas was asked about the *salab* (spoils taken from a slain enemy). He said: "There is no *salab* except from the *nafal* (extraordinary spoils), and in the *nafal*, there is the one-fifth (*khums*)."